Revert r175912, "Add support for coldcc to clang" at John's request.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175936 91177308-0d34-0410-b5e6-96231b3b80d8
Remove the hack that avoided mangling static functions in extern C contexts.

Weather we should give C language linkage to functions and variables with
internal linkage probably depends on how much code assumes it. The standard
says they should have no language linkage, but gcc and msvc assign them
C language linkage.

This commit removes the hack that was preventing the mangling on static
functions declare in extern C contexts. It is an experiment to see if we
can implement the rules in the standard.

If it turns out that many users depend on these functions and variables
having C language linkage, we should change isExternC instead and try
to convince the CWG to change the standard.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175937 91177308-0d34-0410-b5e6-96231b3b80d8
Remove the CFGElement "Invalid" state.

Use Optional<CFG*> where invalid states were needed previously. In the one case
where that's not possible (beginAutomaticObjDtorsInsert) just use a dummy
CFGAutomaticObjDtor.

Thanks for the help from Jordan Rose & discussion/feedback from Ted Kremenek
and Doug Gregor.

Post commit code review feedback on r175796 by Ted Kremenek.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175938 91177308-0d34-0410-b5e6-96231b3b80d8