[modules] Initial support for explicitly loading .pcm files.

Implicit module builds are not well-suited to a lot of build systems. In
particular, they fare badly in distributed build systems, and they lead to
build artifacts that are not tracked as part of the usual dependency management
process. This change allows explicitly-built module files (which are already
supported through the -emit-module flag) to be explicitly loaded into a build,
allowing build systems to opt to manage module builds and dependencies
themselves.

This is only the first step in supporting such configurations, and it should
be considered experimental and subject to change or removal for now.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220359 91177308-0d34-0410-b5e6-96231b3b80d8
Driver: rename Windows to MSVCToolChain

This renames the Windows toolchain to MSVCToolChain.  This is a preparatory step
for adding a CrossWindowsToolChain which uses clang/libc++/(ld/lld) without the
standard GCC toolchain lookup.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220362 91177308-0d34-0410-b5e6-96231b3b80d8