do not warn about unknown pragmas in modes that do not handle them (pr9537)

And refactor to have just one place in code that sets up the empty
pragma handlers.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207758 91177308-0d34-0410-b5e6-96231b3b80d8
revert r207756

There's nothing wrong with the change itself, but
test/Frontend/rewrite-includes-messages.c fails without another
not-yet-committed fix.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207762 91177308-0d34-0410-b5e6-96231b3b80d8
do not use "1" for line marker for the main file

"1" means entering a new file (from a different one), but the main
file is not included from anything (and this would e.g. confuse -Wunused-macros
to not report unused macros in the main file, see pr15610, or also see pr18948).
The line marker is still useful e.g. if the resulting file is renamed or used
via a pipe.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207764 91177308-0d34-0410-b5e6-96231b3b80d8