introduce a new -fheinous-gnu-extensions flag that enables really

really horrible extensions that are disabled by default but that can
be accepted by -fheinous-gnu-extensions (but which always emit a 
warning when enabled).

As our first instance of this, implement PR3788/PR3794, which allows
non-lvalues in inline asms in contexts where lvalues are required. bleh.

llvm-svn: 66910