Add warning for falling off the end of a function that should return a

Add warning for falling off the end of a function that should return a
value.  This is on by default, and controlled by -Wreturn-type (-Wmost
-Wall).  I believe there should be very few false positives, though
the most interesting case would be:

  int() { bar(); }

when bar does:

  bar() { while (1) ; }

Here, we assume functions return, unless they are marked with the
noreturn attribute.  I can envision a fixit note for functions that
never return normally that don't have a noreturn attribute to add a
noreturn attribute.

If anyone spots other false positives, let me know!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@76821 91177308-0d34-0410-b5e6-96231b3b80d8