Implement handling of file entry/exit notifications from GNU

line markers, including maintenance of the virtual include stack.

For something like this:

# 42 "bar.c" 1
# 142 "bar2.c" 1

#warning zappa
# 92 "bar.c" 2
#warning gonzo
# 102 "foo.c" 2
#warning bonkta


we now produce these three warnings:

#1:
In file included from foo.c:3:
In file included from bar.c:42:
bar2.c:143:2: warning: #warning zappa
#warning zappa
 ^

#2:
In file included from foo.c:3:
bar.c:92:2: warning: #warning gonzo
#warning gonzo
 ^

#3:
foo.c:102:2: warning: #warning bonkta
#warning bonkta
 ^

llvm-svn: 63722