swift234

Support build and target configurations

These changes add support for build and target configurations in the compiler.
Build and target configurations, combined with the use of #if/#else/#endif allow
for conditional compilation within declaration and statement contexts.

Build configurations can be passed into the compiler via the new '-D' flag, or
set within the LangOptions class. Target configurations are implicit, and
currently only "os" and "arch" are supported.

Swift SVN r14305