Merging r323053:

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r323053 | dhinton | 2018-01-20 16:29:00 -0800 (Sat, 20 Jan 2018) | 9 lines

[cmake] Don't build Native llvm-config when cross compiling if passed by user.

Summary:
Rename LLVM_CONFIG_EXE to LLVM_CONFIG_PATH, and avoid building it if
passed in by user.  This is the same way CLANG_TABLEGEN and
LLVM_TABLEGEN are handled, e.g., when -DLLVM_OPTIMIZED_TABLEGEN=ON is
passed.

Differential Revision: https://reviews.llvm.org/D41806
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llvm-svn: 332568