GCC overloads -Wwrite-strings just to make it extra confusing. While it

changes language semantics in C and ObjC (which Clang has supported for
a while), in C++ it's the name used for Clang's
-Wdeprecated-writable-strings.

Clang's name is at least less overloaded if still confusing (the string
isn't writable, we just allow converting to a non-const pointer without
warning), so I've left it in place and made the GCC name an alias for
compatibility.

With this I've implemented all the aspects of GCC's -Wwrite-strings I've
encountered which didn't work with Clang.

llvm-svn: 130052