fix Issue 21622 - Ambiguous template instantiation without parens crashes compiler

When a template instantiation matches multiple overloads exactly but is not called with parentheses, the compiler generated a ScopeExp that fell through sideeffect.d.

This fix detects if a ScopeExp resolves to a TemplateInstance with no matching aliasdecl or an OverloadSet, and correctly reports the ambiguity instead of crashing.

Author: TANAY-BARGIR

compiler/src/dmd/sideeffect.d

@@ -8,7 +8,9 @@
  * Documentation:  https://dlang.org/phobos/dmd_sideeffect.html
  * Coverage:    https://codecov.io/gh/dlang/dmd/src/master/compiler/src/dmd/sideeffect.d
  */
+
 module dmd.sideeffect;
+
 import dmd.dtemplate;
 import dmd.dsymbol;
 import dmd.astenums;