add really really trivial #line support, where #line now makes every

location below it report as coming from the #line location.  For example,
with:

#line 92 "foo.h"
#warning blarg!
#warning blarg!

we now emit:

foo.h:92:2: warning: #warning blarg!
#warning blarg!
 ^
foo.h:92:2: warning: #warning blarg!
#warning blarg!
 ^

llvm-svn: 63709