[AST] Remove args from LocationCall

This class initially had args to be generic to future needs. In
particular, I thought that source location introspection should show the
getBeginLoc of CallExpr args and the getArgLoc of
TemplateSpecializationLocInfo etc.  However, that is probably best left
out of source location introspection because it involves node traversal.

If something like this is needed in the future, it can be added in the
future.

Differential Revision: https://reviews.llvm.org/D100688