[clang] [Clang] Correctly handle allocations in the condition of a `if constexpr` (PR #146890)

[Erich Keane via cfe-commits]

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@@ -20629,6 +20629,9 @@ Sema::ConditionResult Sema::ActOnCondition(Scope *S, SourceLocation Loc,
 
   case ConditionKind::ConstexprIf:
     Cond = CheckBooleanCondition(Loc, SubExpr, true);
+    assert(isa<FullExpr>(Cond.get()) &&
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erichkeane wrote:

`Cond` could still be invalid here, right?  so `.get` would be ill-formed?  Should we be doing some sort of `if (Cond.isUsable())` or something? 

https://github.com/llvm/llvm-project/pull/146890