Merging r357701:

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r357701 | mgorny | 2019-04-04 07:21:38 -0700 (Thu, 04 Apr 2019) | 8 lines

[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
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llvm-svn: 359857