Fix Issue 21622: Ambiguous template without parentheses gives 'no effect' error

[shivajigandham]

Fixes #21622

Description

When an ambiguous template function is instantiated without parentheses (e.g. \oo!0;), the compiler previously reported a generic and confusing \has no effect\ error because the expression returned a valid but ambiguous \TemplateInstance\ or \OverExp\ which was then passed to \discardValue().

This PR updates \discardValue()\ in \sideeffect.d\ to explicitly check for \EXP.overloadSet\ and \EXP.scope_\ (when it points to an ambiguous \TemplateInstance/\OverloadSet) and reports a \matches multiple overloads\ error instead, matching the behavior of parenthesized calls.

[dlang-bot]

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dub run digger -- build "master + dmd#22803"