Handle incompatible redeclarations of library builtins better.
Invalid redeclarations of valid explicit declarations shouldn't
take the same path as redeclarations of implicit declarations,
and invalid local extern declarations shouldn't foul things up
for everybody else.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179482 91177308-0d34-0410-b5e6-96231b3b80d8