New frontend: use the target triple to find runtime modules and libraries.

We were accepting -target, but not bothering to recompute the runtime
include path, so we always got lib/swift/macosx (the default target's
platform directory). Fix this by always updating the path after all
options have been parsed.

Tests to come in subsequent commits.

<rdar://problem/16052579>

Swift SVN r13887

lib/Frontend/CompilerInvocation.cpp

@@ -789,5 +789,7 @@ bool CompilerInvocation::parseArgs(ArrayRef<const char *> Args,
     return true;
   }
 
+  updateRuntimeLibraryPath();
+
   return false;
 }