[Sema] Semantic analysis for empty-declaration and attribute-declaration.

Introduce a new AST Decl node "EmptyDecl" to model empty-declaration. Have attributes from attribute-declaration appertain
to the EmptyDecl node by creating the AST representations of these attributes and attach them to the EmptyDecl node so these
attributes can be sema checked just as attributes attached to "normal" declarations.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175900 91177308-0d34-0410-b5e6-96231b3b80d8
Suppress -Wswitch to unbreak the build.

This may need to be fixed more intelligently - I don't have enough context to
be sure what the appropriate fix is right now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175902 91177308-0d34-0410-b5e6-96231b3b80d8
Make sure pragmas don't attach visibility attributes to auto variables with
internal linkage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175903 91177308-0d34-0410-b5e6-96231b3b80d8