Merging r343753:

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r343753 | phosek | 2018-10-03 22:38:53 -0700 (Wed, 03 Oct 2018) | 7 lines

[CMake] Use just basename when copying C++ ABI headers

This avoids duplicate directories when the filename includes path.

Fixes PR39145

Differential Revision: https://reviews.llvm.org/D52762
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llvm-svn: 348322