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

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)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@78019 91177308-0d34-0410-b5e6-96231b3b80d8