Allow a header to be part of multiple modules.

This patch changes two things:

a) Allow a header to be part of multiple modules. The reasoning is that
in existing codebases that have a module-like build system, the same
headers might be used in several build targets. Simple reasons might be
that they defined different classes that are declared in the same
header. Supporting a header as a part of multiple modules will make the
transistion easier for those cases. A later step in clang can then
determine whether the two modules are actually compatible and can be
merged and error out appropriately. The later check is similar to what
needs to be done for template specializations anyway.

b) Allow modules to be stored in a directory tree separate from the
headers they describe.

Review: http://llvm-reviews.chandlerc.com/D1951