We don't want typos in import statements to take down the whole REPL, but we
/do/ want the REPL to be honoring fatal errors that effectively take down the
ASTContext.
This doesn't (yet) apply to the real LLDB REPL, which does not use
SourceFileKind::REPL for its input. The right option to test there is
LangOpts.DebuggerSupport, but that's currently being set for Playgrounds as
well. I've filed <rdar://problem/18090611> for LLDB to adjust their input.
Part of <rdar://problem/17994094>
Swift SVN r21383
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
