Fix issue 19465 - improve 'cannot uniquely infer foreach argument types' diagnostic

Add supplemental error messages explaining why no opApply overload
matched a foreach statement's parameters (parameter count mismatch,
qualifier/mutability mismatch, or explicit type mismatch), instead of
the previous generic error with no further detail.

The added diagnostics are only printed when no more specific error
(e.g. ambiguous overload match) was already reported for the same
failure, to avoid duplicating existing diagnostics.

Author: usefahmed07

compiler/src/dmd/opover.d

@@ -1564,16 +1564,13 @@ private FuncDeclaration findBestOpApplyMatch(Expression ethis, FuncDeclaration f
     return fd_best;
 }
 
-/**
- * Print supplemental diagnostics explaining why no `opApply` overload
- * matched the `foreach` statement's parameters, to clarify the generic
- * "cannot uniquely infer foreach argument types" error (issue 19465).
- * Only call this when no more-specific error (e.g. ambiguity) was
- * already printed for the same failure.
+/******************************
+ * Print supplemental messages explaining why no opApply overload
+ * matched the foreach parameters.
  * Params:
- *      fstart = first opApply overload (start of overload chain)
- *      parameters = foreach parameters, as written (types may be missing)
- *      aggrMod = mutability qualifier of the foreach aggregate
+ *      fstart = first opApply overload
+ *      parameters = foreach parameters
+ *      aggrMod = mutability of the foreach aggregate
  */
 void explainForeachArgMismatch(FuncDeclaration fstart, Parameters* parameters, MOD aggrMod)
 {