Module: add -fprebuilt-module-path to support loading prebuilt modules.

In this mode, there is no need to load any module map and the programmer can
simply use "@import" syntax to load the module directly from a prebuilt
module path. When loading from prebuilt module path, we don't support
rebuilding of the module files and we ignore compatible configuration
mismatches.

rdar://27290316
Differential Revision: http://reviews.llvm.org/D23125

llvm-svn: 279096