Progress towards making isUsed() reflect whether a declaration is odr-used; don't...

Progress towards making isUsed() reflect whether a declaration is odr-used; don't set isUsed for local variables which are referenced in unevaluated contexts.  Make other code use isReferenced() (which basically indicates that a declaration isn't dead) where appropriate.

I was forced to change test/SemaCXX/linkage.cpp because we aren't actually modeling extern "C" in the AST the way that testcase expects; we were not printing a warning only because we skipped the relevant check.  Someone who actually understands the semantics here should fix that.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148158 91177308-0d34-0410-b5e6-96231b3b80d8