swift234

Diagnose self-imports of the module being compiled.

The case where this comes up is when people name their app and framework
targets the same thing, or when they've renamed their test target module
in an attempt to avoid issues with  NSClassFromString and differing
runtime names. We currently do various wrong things when this happens,
so just emit an error instead.

I left a hole for our overlays, which use '@exported import <the-current-module>'
to get at their Clang modules. The previous commit means this can be
replaced by -import-underlying-module, but that doesn't help our tests,
which use -enable-source-import for their overlays. Which we should stop doing.

rdar://problem/21254367

Swift SVN r29440