swift234

Add limited build configuration support for testing against compiler submit versions.  (rdar://problem/16337966)

This is meant to be utilized for a narrow set of scenarios specific to dogfooding our pre-1.0 compiler, so please do not take any dependencies on this.  In fact, I'll be removing this in the next milestone.  (See rdar://problem/16380797.)

Also included - improve error recovery when parsing broken build configuration clauses.

Swift SVN r15694