Improve the diagnostic text for -Wmissing-noreturn to include the name

Improve the diagnostic text for -Wmissing-noreturn to include the name
of the function in question when applicable (that is, not for blocks).
Patch by Joerg Sonnenberger with some stylistic tweaks by me.

When discussing this weth Joerg, streaming the decl directly into the
diagnostic didn't work because we have a pointer-to-const, and the
overload doesn't accept such. In order to make my style tweaks to the
patch, I first changed the overload to accept a pointer-to-const, and
then changed the diagnostic printing layer to also use
a pointer-to-const, cleaning up a gross line of code along the way.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138854 91177308-0d34-0410-b5e6-96231b3b80d8