Add #pragma clang module build/endbuild pragmas for performing a module build

Add #pragma clang module build/endbuild pragmas for performing a module build
as part of a compilation.

This is intended for two purposes:

1) Writing self-contained test cases for modules: we can now write a single
source file test that builds some number of module files on the side and
imports them.

2) Debugging / test case reduction. A single-source testcase is much more
amenable to reduction, compared to a VFS tarball or .pcm files.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305101 91177308-0d34-0410-b5e6-96231b3b80d8