[CMAKE] Find ld64 using xcrun

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.

This is needed in order to get LIT to have the full path to the ld4
executable.

Differential Revision: https://reviews.llvm.org/D22791

rdar://problem/24300926

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@276781 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/config-ix.cmake

@@ -548,8 +548,19 @@ find_program(GOLD_EXECUTABLE NAMES ${LLVM_DEFAULT_TARGET_TRIPLE}-ld.gold ld.gold
 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)