This handles both Clang’s transitive inclusion and the use of
"adapter modules" to augment the Clang modules (e.g. Foundation.swift),
at the cost of a bit more memory (used to wrap all the Clang modules
in ClangModule objects). This is paving the way for making Sema
independent of ClangImporter.
Swift SVN r6698
This makes it very clear who is depending on special behavior at the
module level. Doing isa<ClangModule> now requires a header import; anything
more requires actually linking against the ClangImporter library.
If the current source file really can't import ClangModule.h, it can
still fall back to checking against the DeclContext's getContextKind()
(and indeed AST currently does in a few places).
Swift SVN r6695
