[modules] Instead of storing absolute paths in a .pcm file, store the path to

[modules] Instead of storing absolute paths in a .pcm file, store the path to
the root of the module and use paths relative to that directory wherever
possible. This is a step towards allowing explicit modules to be relocated
without being rebuilt, which is important for some kinds of distributed builds,
for good paths in diagnostics, and for appropriate .d output.

llvm-svn: 223443