overload resolution: Ingore CONSTness for RTL procedures without RTL_CONST

(closes #735)

This is a regression from 8d8df7ad. Before that commit, the code contained
two calls to symbCheckConstAssign(), the first guarded by a
	if( typeGetConstMask( param_dtype ) ) then
and the second guarded by
	if( (symbGetIsRTL( parent ) = FALSE) or (symbGetIsRTLConst( param )) ) then
Given that none of the RTL functions had CONSTs on their parameters,
all were redirected to the second check, causing them to be handled
correctly. The commit changed the code though, such that the first call
to symbCheckConstAssign() was no longer guarded this way.
hCalcTypesDiff(): Fix comment typo
overload resolution: Combine and fix redundant CONSTness checks

(closes #728)

1. The first symbCheckConstAssign() did not consider parameter mode

2. The entire code path/logic seemed weird/wrong - e.g. two calls to
symbCheckConstAssign(), one outside the "type_is_compatible" guard.

This already doesn't make sense as it would allow incompatible types to
match during overload resolution if they match based on CONSTness alone.

Also, the part of code using const_matches for the second
symbCheckConstAssign() call does not make sense, as it can never be reached
if the types are compatible, because then the first constness check will
already have determined the match. Thus for now that is removed too, though
I think it can be fixed in the future.