Implement basic support for private headers in frameworks. In essence,
when we load a module map (module.map) from a directory, also load a
private module map (module_private.map) for that directory,

Implement basic support for private headers in frameworks. In essence,
when we load a module map (module.map) from a directory, also load a
private module map (module_private.map) for that directory, if
present. That private module map can inject a new submodule that
captures private headers.

llvm-svn: 146012

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