Add specific warning flags for GNU ext in Sema.

This patch adds the following, more specific warning flags:

gnu-anonymous-struct
gnu-compound-literal-initializer
gnu-empty-struct
gnu-flexible-array-initializer
gnu-flexible-array-union-member
gnu-folding-constant
redeclared-class-member
gnu-redeclared-enum
gnu-union-cast
gnu-variable-sized-type-not-at-end

Patch by Peter Lewis.

llvm-svn: 190972