[cmake] When compiling swift libraries that are not pure swift on macOS be sure to set the linker language to cxx.

I already did this for executables. The reason why we must do this is that right
now there are bugs in cmake's swift support that makes it so that one can not
use swiftc to link targets with mixed c/c++/swift content even if the swift
content is indirectly added via linking. To work around this we must also ensure
that all libraries with mixed c/c++/swift objects link using clang. As an
additional side-effect of this, we must pass to the clang linker -L flags that
swiftc would normally provide for the linker. The two cases where I needed to do
this were:

1. -L path for the compatibility libraries. This path points into the toolchain
where these live. This is important since otherwise we will fail to link given
the minimum deployment target that swift (both host/stdlib) target today (10.9).

2. -L path to the SDK. This path points to the SDKs lib/swift/${HOST_PLATFORM}
for swiftCore and friends.

That being said, we still want people to be able to link pure swift libraries
using swiftc, so we introduce an option called PURE_SWIFT to add_host_library
that preserves this and is used to also set
IGNORE_LLVM_UPDATE_COMPILE_FLAGS (which arguably should have been called
PURE_SWIFT).

Author: gottesmm

cmake/modules/AddSwift.cmake

@@ -390,7 +390,7 @@ endfunction()
 #     [SHARED]
 #     [STATIC]
 #     [OBJECT]
-#     [IGNORE_LLVM_UPDATE_COMPILE_FLAGS]
+#     [PURE_SWIFT]
 #     [LLVM_LINK_COMPONENTS comp1 ...]
 #     source1 [source2 source3 ...])
 #
@@ -409,10 +409,9 @@ endfunction()
 # LLVM_LINK_COMPONENTS
 #   LLVM components this library depends on.
 #
-# IGNORE_LLVM_UPDATE_COMPILE_FLAGS
-#   If set do not use llvm_update_compile_flags to generate cflags/etc.  This is
-#   generally used when compiling a mixed c/c++/swift library and one wants to
-#   disable this for the swift part.
+# PURE_SWIFT
+#   This has two effects if set: we do not use llvm_update_compile_flags to
+#   generate cflags/etc and we leave the linking mode of the library as swift.
 #
 # source1 ...
 #   Sources to add into this library.
@@ -421,7 +420,7 @@ function(add_swift_host_library name)
         SHARED
         STATIC
         OBJECT
-        IGNORE_LLVM_UPDATE_COMPILE_FLAGS)
+        PURE_SWIFT)
   set(single_parameter_options)
   set(multiple_parameter_options
         LLVM_LINK_COMPONENTS)
@@ -467,7 +466,7 @@ function(add_swift_host_library name)
 
   add_library(${name} ${libkind} ${ASHL_SOURCES})
   add_dependencies(${name} ${LLVM_COMMON_DEPENDS})
-  if (NOT ASHL_IGNORE_LLVM_UPDATE_COMPILE_FLAGS)
+  if (NOT ASHL_PURE_SWIFT)
     llvm_update_compile_flags(${name})
   endif()
   swift_common_llvm_config(${name} ${ASHL_LLVM_LINK_COMPONENTS})
@@ -528,6 +527,12 @@ function(add_swift_host_library name)
       NO_SONAME YES)
   endif()
 
+  # Always link as CXX even if we have swift content unless we only contain
+  # swift content signaled via us being marked "PURE_SWIFT".
+  if (NOT ASHL_PURE_SWIFT)
+    set_target_properties(${name} PROPERTIES LINKER_LANGUAGE CXX)
+  endif()
+
   if(${SWIFT_HOST_VARIANT_SDK} IN_LIST SWIFT_APPLE_PLATFORMS)
     target_link_options(${name} PRIVATE
       "LINKER:-compatibility_version,1")
@@ -536,8 +541,24 @@ function(add_swift_host_library name)
         "LINKER:-current_version,${SWIFT_COMPILER_VERSION}")
     endif()
 
-    # For now turn off in swift targets, debug info if we are compiling a static
-    # library.
+    # If we found a swift compiler and are going to use swift code in swift
+    # host side tools but link with clang, add the appropriate -L paths so we
+    # find all of the necessary swift libraries on Darwin.
+    if (NOT ASHL_PURE_SWIFT)
+      if (CMAKE_Swift_COMPILER)
+        # Add in the SDK directory for the host platform and add an rpath.
+        target_link_directories(${name} PRIVATE
+          ${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_ARCH_${SWIFT_HOST_VARIANT_ARCH}_PATH}/usr/lib/swift)
+        # Add in the toolchain directory so we can grab compatibility libraries
+        get_filename_component(TOOLCHAIN_BIN_DIR ${CMAKE_Swift_COMPILER} DIRECTORY)
+        get_filename_component(TOOLCHAIN_LIB_DIR "${TOOLCHAIN_BIN_DIR}/../lib/swift/macosx" ABSOLUTE)
+        target_link_directories(${name} PUBLIC ${TOOLCHAIN_LIB_DIR})
+      endif()
+    endif()
+
+    # For now turn off on Darwin swift targets, debug info if we are compiling a static
+    # library and set up an rpath so that if someone works around this by using
+    # shared libraries that in the short term we can find shared libraries.
     if (ASHL_STATIC)
       target_compile_options(${name} PRIVATE $<$<COMPILE_LANGUAGE:Swift>:-gnone>)
     endif()
@@ -605,9 +626,23 @@ function(add_swift_host_tool executable)
       JOB_POOL_LINK swift_link_job_pool)
   endif()
   if(${SWIFT_HOST_VARIANT_SDK} IN_LIST SWIFT_APPLE_PLATFORMS)
+    # Lists of rpaths that we are going to add to our executables.
+    #
+    # Please add each rpath separately below to the list, explaining why you are
+    # adding it.
+    set(RPATH_LIST)
+
+    # We also want to be able to find libraries from the base toolchain
+    # directory. This is so swiftc can rely on its own host side dylibs that may
+    # contain swift content.
+    list(APPEND RPATH_LIST "@executable_path/../lib")
+
+    # Also include the swift specific resource dir in our rpath.
+    list(APPEND RPATH_LIST "@executable_path/../lib/swift/${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_LIB_SUBDIR}")
+
     set_target_properties(${executable} PROPERTIES
       BUILD_WITH_INSTALL_RPATH YES
-      INSTALL_RPATH "@executable_path/../lib/swift/${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_LIB_SUBDIR}")
+      INSTALL_RPATH "${RPATH_LIST}")
   endif()
 
   llvm_update_compile_flags(${executable})