Warn on duplicate function specifier

This patch fixes PR8264. Duplicate qualifiers already are diagnozed,
now the same diagnostics is issued for duplicate function specifiers.

Differential Revision: http://llvm-reviews.chandlerc.com/D2025


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194559 91177308-0d34-0410-b5e6-96231b3b80d8