[CMake] Prefer --ld-path when supported

[keith]

Summary

This allows relative linker paths, and avoids the warning for path usage
with -fuse-ld. When the compiler doesn't support it this falls back to
the previous behavior.

llvm/cmake/modules/HandleLLVMOptions.cmake

if( LLVM_ENABLE_LLD )
   # compiler option.
   if ( NOT MSVC )
     set(LLVM_USE_LINKER "lld")
   endif()
 endif()
 
 if( LLVM_USE_LINKER )
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fuse-ld=${LLVM_USE_LINKER}")
+  set(linker_arg "--ld-path")
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} ${linker_arg}=${LLVM_USE_LINKER}")
   check_cxx_source_compiles("int main() { return 0; }" CXX_SUPPORTS_CUSTOM_LINKER)
   if ( NOT CXX_SUPPORTS_CUSTOM_LINKER )
-    message(FATAL_ERROR "Host compiler does not support '-fuse-ld=${LLVM_USE_LINKER}'")
+    set(linker_arg "-fuse-ld")
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} ${linker_arg}=${LLVM_USE_LINKER}")
+    check_cxx_source_compiles("int main() { return 0; }" CXX_SUPPORTS_CUSTOM_LINKER)
+  endif()
+  if ( NOT CXX_SUPPORTS_CUSTOM_LINKER )
+    message(FATAL_ERROR "Host compiler does not support '--ld-path=${LLVM_USE_LINKER}' or '-fuse-ld=${LLVM_USE_LINKER}'")
   endif()
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
-  append("-fuse-ld=${LLVM_USE_LINKER}"
+  append("${linker_arg}=${LLVM_USE_LINKER}"
     CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
 endif()
 
 if( LLVM_ENABLE_PIC )
   if( XCODE )
     # Xcode has -mdynamic-no-pic on by default, which overrides -fPIC. I don't
     # know how to disable this, so just force ENABLE_PIC off for now.
     message(WARNING "-fPIC not supported with Xcode.")