Don't call getMostRecentDecl when we know we have it.

On a Release build this takes the testcase in pr18055 from 0m3.892s to 0m1.452s.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195768 91177308-0d34-0410-b5e6-96231b3b80d8
Replacing a custom diagnostic with a more standard one. No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195770 91177308-0d34-0410-b5e6-96231b3b80d8