[CodeGen] Always use MSVC personality for windows-msvc targets

The windows-msvc target is meant to be ABI compatible with MSVC,
including the exception handling. Ensure that a windows-msvc triple
always equates to the MSVC personality being used.

This mostly affects the GNUStep and ObjFW Obj-C runtimes. To the best of
my knowledge, those are normally not used with windows-msvc triples. I
believe WinObjC is based on GNUStep (or it at least uses libobjc2), but
that also takes the approach of wrapping Obj-C exceptions in C++
exceptions, so the MSVC personality function is the right one to use
there as well.

Differential Revision: https://reviews.llvm.org/D47862

llvm-svn: 334253