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.

llvm-svn: 305101