build: allow experimental selection of lld

Add an option which the user can specify to switch to the lld linker.  Although
this linker is still nascent, it is interesting to permit linking with this.  It
is also an alternative to the BFD linker for COFF targets.  It will allow for a
cross-compilation story for Windows on non-Windows targets.

Author: compnerd

CMakeLists.txt

@@ -110,6 +110,10 @@ set(SWIFT_COMPILER_VERSION "" CACHE STRING
 set(CLANG_COMPILER_VERSION "" CACHE STRING
     "The internal version of the Clang compiler")
 
+# Indicate whether Swift should attempt to use the lld linker.
+set(SWIFT_ENABLE_LLD_LINKER FALSE CACHE BOOL
+    "Enable using the lld linker when available")
+
 # Indicate whether Swift should attempt to use the gold linker.
 # This is not used on Darwin.
 set(SWIFT_ENABLE_GOLD_LINKER FALSE CACHE BOOL

cmake/modules/AddSwift.cmake

@@ -888,6 +888,9 @@ function(_add_swift_library_single target name)
      "${SWIFT_SDK_${SWIFTLIB_SINGLE_SDK}_OBJECT_FORMAT}" STREQUAL "ELF")
     list(APPEND link_flags "-fuse-ld=gold")
   endif()
+  if (SWIFT_ENABLE_LLD_LINKER)
+    list(APPEND link_flags "-fuse-ld=lld")
+  endif()
 
   # Configure plist creation for OS X.
   set(PLIST_INFO_PLIST "Info.plist" CACHE STRING "Plist name")
@@ -1530,6 +1533,9 @@ function(_add_swift_executable_single name)
      "${SWIFT_SDK_${SWIFTEXE_SINGLE_SDK}_OBJECT_FORMAT}" STREQUAL "ELF")
     list(APPEND link_flags "-fuse-ld=gold")
   endif()
+  if(SWIFT_ENABLE_LLD_LINKER)
+    list(APPEND link_flags "-fuse-ld=lld")
+  endif()
 
   # Find the names of dependency library targets.
   #

cmake/modules/AddSwiftUnittests.cmake

@@ -51,6 +51,10 @@ function(add_swift_unittest test_dirname)
     set_property(TARGET "${test_dirname}" APPEND_STRING PROPERTY
       LINK_FLAGS " -fuse-ld=gold")
   endif()
+  if(SWIFT_ENABLE_LLD_LINKER)
+    set_property(TARGET "${test_dirname}" APPEND_STRING PROPERTY
+      LINK_FLAGS " -fuse-ld=lld")
+  endif()
 endfunction()