Both uniform and varying function pointers are supported; when a function
is called through a varying function pointer, each unique function pointer
value across the running program instances is called once for the set of
active program instances that want to call it.
This code previously lived in FunctionCallExpr but is now part
of FunctionSymbolExpr. This change doesn't change any current
functionality, but lays groundwork for function pointers in
the language, where we'll want to do function call overload
resolution at other times besides when a function call is
actually being made.
