[CMAKE] Find ld64 using xcrun

[bruno]

Summary

Given similar reasons from r276710, ld64 scrubs DYLD_* environment if called from the shim executable /usr/bin/ld.

Add support for finding ld64 via xcrun.

To get http://lab.llvm.org:8080/green/job/clang-stage2-cmake-RgSan_check green, we need to pass DYLD_INSERT_LIBRARIES=<PATH_TO>/libclang_rt.asan_osx_dynamic.dylib to ld64, which currently won't work because we're invoking /usr/bin/ld.

[beanz]

@bruno, we shouldn't need to do this for LD64 because the DARWIN_LTO_LIBRARY option passes the LTO library into ld64 via the linker flags.

[bruno]

Makes sense for the libLTO case, however, we need lit to be able to pass down DYLD_INSERT_LIBRARIES=<PATH_TO>/libclang_rt.asan_osx_dynamic.dylib to ld64, which won't work for /usr/bin/ld. LIT obtain the ld64 executable through CMAKE:

test/lit.site.cfg.in:config.ld64_executable = "@LD64_EXECUTABLE@"

I'm avoiding to gather ld64 path once again in LIT because it seems clean to use the same LD64, suggestions?

[beanz]

Ah! I understand what you're doing. This LGTM.

[bruno]

Thanks Chris,

Committed r276781

cmake/config-ix.cmake

 set(LLVM_BINUTILS_INCDIR "" CACHE PATH
   "PATH to binutils/include containing plugin-api.h for gold plugin.")
 
-if(APPLE)
-  find_program(LD64_EXECUTABLE NAMES ld DOC "The ld64 linker")
+if(CMAKE_HOST_APPLE AND APPLE)
+  if(CMAKE_XCRUN)
+    execute_process(COMMAND ${CMAKE_XCRUN} -find ld
+      OUTPUT_VARIABLE LD64_EXECUTABLE
+      OUTPUT_STRIP_TRAILING_WHITESPACE)
+  else()
+    find_program(LD64_EXECUTABLE NAMES ld DOC "The ld64 linker")
+  endif()
+
+  if(LD64_EXECUTABLE)
+    set(LD64_EXECUTABLE ${LD64_EXECUTABLE} CACHE PATH "ld64 executable")
+    message(STATUS "Found ld64 - ${LD64_EXECUTABLE}")
+  endif()
 endif()
 
 include(FindOCaml)