Tweak how -Wunused-value interacts with macros

1. Make the warning more strict in C mode. r172696 added code to suppress
   warnings from macro expansions in system headers, which checks
   `SourceMgr.isMacroBodyExpansion(E->IgnoreParens()->getExprLoc())`. Consider
   this snippet:

   #define FOO(x) (x)
   void f(int a) {
     FOO(a);
   }

   In C, the line `FOO(a)` is an `ImplicitCastExpr(ParenExpr(DeclRefExpr))`,
   while it's just a `ParenExpr(DeclRefExpr)` in C++. So in C++,
   `E->IgnoreParens()` returns the `DeclRefExpr` and the check tests the
   SourceLoc of `a`. In C, the `ImplicitCastExpr` has the effect of checking the
   SourceLoc of `FOO`, which is a macro body expansion, which causes the
   diagnostic to be skipped. It looks unintentional that clang does different
   things for C and C++ here, so use `IgnoreParenImpCasts` instead of
   `IgnoreParens` here. This has the effect of the warning firing more often
   than previously in C code – it now fires as often as it fires in C++ code.

2. Suppress the warning if it would warn on `UNREFERENCED_PARAMETER`.
   `UNREFERENCED_PARAMETER` is a commonly used macro on Windows and it happens
   to uselessly trigger -Wunused-value. As discussed in the thread
   "rfc: winnt.h's UNREFERENCED_PARAMETER() vs clang's -Wunused-value" on
   cfe-dev, fix this by special-casing this specific macro. (This costs a string
   comparison and some fast-path lexing per warning, but the warning is emitted
   rarely. It fires once in Windows.h itself, so this code runs at least once
   per TU including Windows.h, but it doesn't run hundreds of times.)

http://reviews.llvm.org/D13969

llvm-svn: 251441