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/Sema/ConstraintSystem.cpp

@@ -1460,7 +1460,7 @@ void ConstraintSystem::resolveOverload(ConstraintLocator *locator,
       auto boundFunctionType = boundType->getAs<AnyFunctionType>();
         
       if (boundFunctionType &&
-          CD->isBodyThrowing() != boundFunctionType->throws()) {
+          CD->hasThrows() != boundFunctionType->throws()) {
         boundType = FunctionType::get(boundFunctionType->getInput(),
                                       boundFunctionType->getResult(),
                                       boundFunctionType->getExtInfo().