Attempt to appease msc buildbot

It was timing out on this test, but for reasons unrelated to the
specific bug it was testing for. Randomly breaking in gdb with `clang
-target i686-windows -fmsc-version=1700` reveals *many* frames from
MicrosoftCXXNameMangler. So, it would seem that some caching is needed
there, as well...

Fingers crossed that specifying a triple is sufficient to work around
this.

llvm-svn: 310444