swift234

Allow ASTContext::getStdlibModule to load the stdlib if it isn't present.

This is better than requiring people to go through Ctx.getModule() using
Ctx.StdlibModuleName. There aren't that many cases of this, but they
shouldn't be using a completely different API.

The default behavior remains the same: if the standard library has not been
loaded, null will be returned.

Refinement of r18796, which modified the behavior of SourceFiles to assume
that the standard library had already been loaded.

Swift SVN r18813