Per advice that Doug Gregor gave me several months ago, clean up the

implementation of '#pragma unused' by not constructing intermediate
DeclRefExprs, but instead do the name lookup directly.  The
implementation is greatly simplified.

Along the way, degrade '#pragma unused(undeclaredvariable)' to a
warning instead of being a hard error.  This implements:

<rdar://problem/6761874> [sema] allow #pragma unused to reference undefined variable (with warning)

llvm-svn: 78019