a subprocess invocation which is pretty significant on Windows. It also
likely saves a bunch of thrashing the host machine needlessly. Finally
it makes the tests much more predictable and less dependent on the host.
For example 'header_lookup1.c' was passing '-fno-ms-extensions' just to
thwart the host detection adding it into the compilation. By runnig CC1
directly we don't have to deal with such oddities.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199308 91177308-0d34-0410-b5e6-96231b3b80d8