Merging r341244:

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r341244 | tstellar | 2018-08-31 22:15:31 +0200 (Fri, 31 Aug 2018) | 11 lines

lit: Use sys.executable for executing builtin commands

Summary:
The python executable may not exist on all systems so use sys.executable
instead.

Reviewers: ddunbar, stella.stamenova

Subscribers: delcypher, llvm-commits

Differential Revision: https://reviews.llvm.org/D51511
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_70@341349 91177308-0d34-0410-b5e6-96231b3b80d8