Merging r312748:

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r312748 | jroelofs | 2017-09-07 15:01:25 -0700 (Thu, 07 Sep 2017) | 10 lines

Fix validation of the -mthread-model flag in the Clang driver

The ToolChain class validates the -mthread-model flag in the constructor which
doesn't work correctly since the thread model methods are virtual methods. The
check is moved into Clang::ConstructJob() when constructing the internal
command line.

https://reviews.llvm.org/D37496

Patch by: Ian Tessier!
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llvm-svn: 318346