AST: Add Throws flag and ThrowsLoc to AbstractFunctionDecl

The verifier now asserts that Throws, ThrowsLoc and isBodyThrowing()
match up.

Also, add /*Label=*/ comments where necessary to make the long argument
lists easier to read, and cleaned up some inconsistent naming conventions.

I caught a case where ClangImporter where we were passing in a loc as
StaticLoc instead of FuncLoc, but probably this didn't affect anything.

lib/Serialization/Serialization.cpp

@@ -2476,6 +2476,7 @@ void Serializer::writeDecl(const Decl *D) {
                            fn->isObjC(),
                            fn->isMutating(),
                            fn->hasDynamicSelf(),
+                           fn->hasThrows(),
                            fn->getParameterLists().size(),
                            addTypeRef(fn->getType()),
                            addTypeRef(fn->getInterfaceType()),
@@ -2595,6 +2596,7 @@ void Serializer::writeDecl(const Decl *D) {
                                   ctor->isImplicit(),
                                   ctor->isObjC(),
                                   ctor->hasStubImplementation(),
+                                  ctor->hasThrows(),
                                   getStableCtorInitializerKind(
                                     ctor->getInitKind()),
                                   addTypeRef(ctor->getType()),