[-Wunreachable-code] Simplify and broad -Wunreachable-code-return, including nontrivial returns.

The exception is return statements that include control-flow,
which are clearly doing something "interesting".

99% of the cases I examined for -Wunreachable-code that fired
on return statements were not interesting enough to warrant
being in -Wunreachable-code by default.  Thus the move to
include them in -Wunreachable-code-return.

This simplifies a bunch of logic, including removing the ad hoc
logic to look for std::string literals.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204307 91177308-0d34-0410-b5e6-96231b3b80d8
[-Wunreachable-code] constexpr functions can be used as configuration values.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204308 91177308-0d34-0410-b5e6-96231b3b80d8