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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@66910 91177308-0d34-0410-b5e6-96231b3b80d8