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
