[llvm] [cmake] Add additional headers only if they exist

Modify the add_header_files_for_glob() function to only add files
that do exist, rather than all matches of the glob.  This fixes CMake
error when one of the include directories (which happen to include
/usr/include) contain broken symlinks.

Differential Revision: https://reviews.llvm.org/D59632

llvm-svn: 357701