A static_assert declaration cannot be a template; adding the diagnostic for this instead of silently accepting and producing possibly-unexpected behavior.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214770 91177308-0d34-0410-b5e6-96231b3b80d8
Nit picky const-correctness with auto. No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214773 91177308-0d34-0410-b5e6-96231b3b80d8