[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