Experiment with making -Wunreachable-code more immediately useful by restricting warnings to those issued in the main file.

This warning has a whole bunch of known false positives, much of them due
to code that is "sometimes unreachable".  This can caused by code that
is conditionally generated by the preprocessor, branches that are defined
in terms of architecture-specific details (e.g., the size of a type), and
so on.  While these are all good things to address one by one, the reality
is that this warning has received little love lately.  By restricting
its purvue, we can focus on the top issues effecting main files, which
should be smaller, and then gradually widen the scope.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201607 91177308-0d34-0410-b5e6-96231b3b80d8