Merging r366513:

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r366513 | lanza | 2019-07-19 01:40:23 +0200 (Fri, 19 Jul 2019) | 9 lines

[cmake] Update NATIVE build variables to account for standalone changes

Summary:
LLDB_PATH_TO_{CLANG,LLVM}_BUILD were removed and replaced with
{LLVM,Clang}_DIR. Adjust the NATIVE build to account for this.

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D64959
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llvm-svn: 368268