Implement the -Wstrict-prototypes warning

This commit fixes PR20796. It implements the C only -Wstrict-prototypes warning.
Clang now emits a warning for function declarations which have no parameters
specified and for K&R function definitions with more than 0 parameters that are
not preceded by a previous prototype declaration.

The patch was originally submitted by Paul Titei!

rdar://15060615

Differential Revision: https://reviews.llvm.org/D16533

llvm-svn: 288896