swift234

[ClangImporter] Check for the existence of a submodule before loading it.

This regresses a bit on the diagnostic when a submodule is missing, as well
as missing out on Clang's automatic typo correction, but it does avoid putting
the Clang CompilerInstance into an error state, which is important for the
REPL. It's also not entirely clear that Clang had the right behavior in this
case either---we'd get a module back, but if the particular submodule we
asked for didn't exist we might get its parent. We can revisit this later.

Prerequisite for <rdar://problem/17994094>.

Swift SVN r21333