Merge function-return.c into function.c

Fix PR2790 by making a warning an EXTWARN instead of EXTENSION.
Add a new EXTENSION warning for "return (some void expression);"


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@61187 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed