Sema: provide an extension warning for enable_if

Clang implements an enable_if attribute as an extension.  Hook up `-Wpedantic`
to issue an extension usage warning when __enable_if__ is used.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@261192 91177308-0d34-0410-b5e6-96231b3b80d8