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!

llvm-svn: 312748