ccc: Implement support clang PTH using gcc PCH style interface.

This requires some hackery, as gcc's PCH mechanism changes behavior,
whereas while PTH is simply a cache. Notably:

 - Automatically cause clang to load a .pth file if we find one that
   matches a command line -include argument (similar to how gcc
   looks for .gch files).

 - When generating precompiled headers, translate the suffix from .gch
   to .pth (so we do not conflict with actual gcc PCH files).

 - When generating precompiled headers, copy the input header to the
   same location as the output PTH file. This is necessary because gcc
   supports -include xxx.h even if xxx.h doesn't exist, but for clang
   we need to actually have the contents of this file available.

llvm-svn: 62246