[CMake] Store path to vendor-specific headers in clang-headers target property

Summary:
LLDB.framework wants a copy these headers. With this change LLDB can easily glob for the list of files:
```
get_target_property(clang_include_dir clang-headers RUNTIME_OUTPUT_DIRECTORY)
file(GLOB_RECURSE clang_vendor_headers RELATIVE ${clang_include_dir} "${clang_include_dir}/*")
```

By default `RUNTIME_OUTPUT_DIRECTORY` is unset for custom targets like `clang-headers`.

Reviewers: aprantl, JDevlieghere, davide, friss, dexonsmith

Reviewed By: JDevlieghere

Subscribers: mgorny, #lldb, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D55128

llvm-svn: 348116

Author: weliveindetail

clang/lib/Headers/CMakeLists.txt

@@ -144,7 +144,7 @@ foreach( f ${files} ${cuda_wrapper_files} )
   list(APPEND out_files ${dst})
 endforeach( f )
 
-add_custom_command(OUTPUT ${output_dir}/arm_neon.h 
+add_custom_command(OUTPUT ${output_dir}/arm_neon.h
   DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/arm_neon.h
   COMMAND ${CMAKE_COMMAND} -E copy_if_different ${CMAKE_CURRENT_BINARY_DIR}/arm_neon.h ${output_dir}/arm_neon.h
   COMMENT "Copying clang's arm_neon.h...")
@@ -156,7 +156,9 @@ add_custom_command(OUTPUT ${output_dir}/arm_fp16.h
 list(APPEND out_files ${output_dir}/arm_fp16.h)
 
 add_custom_target(clang-headers ALL DEPENDS ${out_files})
-set_target_properties(clang-headers PROPERTIES FOLDER "Misc")
+set_target_properties(clang-headers PROPERTIES
+  FOLDER "Misc"
+  RUNTIME_OUTPUT_DIRECTORY "${output_dir}")
 
 install(
   FILES ${files} ${CMAKE_CURRENT_BINARY_DIR}/arm_neon.h