Introduce a FunctionDecl::getReturnTypeSourceRange() utility

This source range is useful for all kinds of diagnostic QOI and refactoring
work, so let's make it more discoverable.

This commit also makes use of the new function to enhance various diagnostics
relating to return types and resolves an old FIXME.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212154 91177308-0d34-0410-b5e6-96231b3b80d8