swift234

[ClangImporter] Test for a module's presence before trying to load it.

This makes sure that the Clang CompilerInstance doesn't think that we've seen
an error just because we were checking for the existence of a module, and
therefore will not break the world when the module is absent.

This fixes an issue where loading PlaygroundLogger broke static inline
functions because PlaygroundLogger has no Objective-C module.
<rdar://problem/18007508>

It also lays groundwork for not treating a missing module as a fatal error
in REPL modes. To do that properly, however, we should figure out how to do
this for submodules as well. <rdar://problem/17994094>

Swift SVN r21187