[SEMA] split ExtWarn dupl-decl-spec's into Extension and ExtWarn

Summary:
For types deduced from typedef's and typeof's, don't warn for duplicate
declaration specifiers in C90 unless -pedantic.

Create a third diagnostic type for duplicate declaration specifiers.
Previously, we had an ExtWarn and a Warning. This change adds a third,
Extension, which only warns when -pedantic is set, staying silent
otherwise.

Fixes PR32985.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D52849

llvm-svn: 343740