Add a warning to diagnose statements in C++ like "*(volatile int*)x;". ...

Add a warning to diagnose statements in C++ like "*(volatile int*)x;".  Conceptually, this is part of -Wunused-value, but I added a separate flag -Wunused-volatile-lvalue so it doesn't get turned off by accident with -Wno-unused-value.  I also made a few minor improvements to existing unused value warnings in the process.  <rdar://problem/11516811>.

llvm-svn: 157362