[cmake] Add the option SWIFT_BUILD_RUNTIME_WITH_HOST_LIBLTO.

Currently when we build with bitcode, we use the liblto that we just built,
despite the fact that we just compiled the runtime with the host clang. Really,
we should be consistent.

In the short term to unblock LTO (which wants to use the host liblto), provide
an option to use the host liblto but leave it off so that the normal behavior is
preserved.

Author: gottesmm

CMakeLists.txt

@@ -93,6 +93,9 @@ option(SWIFT_STDLIB_ASSERTIONS
 option(SWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER
        "Use the host compiler and not the internal clang to build the swift runtime"
        FALSE)
+option(SWIFT_BUILD_RUNTIME_WITH_HOST_LIBLTO
+       "Use the host liblto and not the internal liblto to build the swift runtime"
+       FALSE)
 
 set(SWIFT_ANALYZE_CODE_COVERAGE FALSE CACHE STRING
     "Build Swift with code coverage instrumenting enabled [FALSE, NOT-MERGED, MERGED]")

cmake/modules/AddSwift.cmake

@@ -506,7 +506,10 @@ function(_add_swift_library_single target name)
     if("${SWIFTLIB_SINGLE_SDK}" STREQUAL "IOS" OR "${SWIFTLIB_SINGLE_SDK}" STREQUAL "TVOS" OR "${SWIFTLIB_SINGLE_SDK}" STREQUAL "WATCHOS")
       list(APPEND SWIFTLIB_SINGLE_C_COMPILE_FLAGS "-fembed-bitcode")
       list(APPEND SWIFTLIB_SINGLE_SWIFT_COMPILE_FLAGS "-embed-bitcode")
-      list(APPEND SWIFTLIB_SINGLE_LINK_FLAGS "-Xlinker" "-bitcode_bundle" "-Xlinker" "-bitcode_hide_symbols" "-Xlinker" "-lto_library" "-Xlinker" "${LLVM_LIBRARY_DIR}/libLTO.dylib")
+      list(APPEND SWIFTLIB_SINGLE_LINK_FLAGS "-Xlinker" "-bitcode_bundle" "-Xlinker" "-bitcode_hide_symbols")
+      if (NOT SWIFT_BUILD_RUNTIME_WITH_HOST_LIBLTO)
+        list(APPEND SWIFTLIB_SINGLE_LINK_FLAGS "-Xlinker" "-lto_library" "-Xlinker" "${LLVM_LIBRARY_DIR}/libLTO.dylib")
+      endif()
     endif()
   endif()