-Wshadow should only warn about parameter declarations when we're

-Wshadow should only warn about parameter declarations when we're
entering a function or block definition, not on every single declaration.
Unfortunately we don't have previous-lookup results around when it's time
to make this decision, so we have to redo the lookup.  The alternative is
to use delayed diagnostics.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@99172 91177308-0d34-0410-b5e6-96231b3b80d8