Fixed the AST importer ot use redeclaration contexts properly.

To ensure that we don't import duplicates, the ASTImporter often
checks the DeclContext for a Decl before importing it to see if
a Decl with the same name is already present.

This is problematic if the Decl is inside a transparent context
like an extern "C" block.  Lookup isn't allowed on such contexts,
and in fact they assert() if you do that.  So instead we look at
the redeclaration context -- the containing context that can be
used safely for these kinds of checks -- instead.
Reinstate r223753, reverted in r223759 due to breakage of clang-tools-extra.
Original commit message:

[modules] Add experimental -fmodule-map-file-home-is-cwd flag to -cc1.

For files named by -fmodule-map-file=, and files found by 'extern module'
directives, this flag specifies that we should resolve filenames relative to
the current working directory rather than relative to the directory in which
the module map file resides. This is aimed at fixing path handling, in
particular for relative -I paths, when building modules that represent
components of the current project (rather than libraries installed on the
current system, which the current project has as dependencies, where we'd
typically expect the module map files to be looked up implicitly).
Update to match clang r223913.