Sema: Static redeclaration after extern declarations is a Microsoft Extension

CL permits static redeclarations to follow extern declarations.  The
storage specifier on the latter declaration has no effect.

This fixes PR20034.

Differential Revision: http://reviews.llvm.org/D4149

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211238 91177308-0d34-0410-b5e6-96231b3b80d8