Merging r371733:

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r371733 | mgorny | 2019-09-12 06:06:12 -0700 (Thu, 12 Sep 2019) | 7 lines

[clang] [unittest] Import LLVMTestingSupport if necessary

Add LLVMTestingSupport directory from LLVM_MAIN_SRC_DIR when building
clang stand-alone and LLVMTestingSupport library is not present.  This
is needed to fix stand-alone builds without clang-tools-extra.

Differential Revision: https://reviews.llvm.org/D67452
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