[llvm] [LLVM Demangler] Set `InConstraintExpr` to `true` when demangling a c… (PR #107385)

[Ilya Biryukov via llvm-commits]

================
@@ -5737,6 +5737,7 @@ Node *AbstractManglingParser<Derived, Alloc>::parseTemplateParamDecl(
   }
 
   if (consumeIf("Tk")) {
+    ScopedOverride<bool> SaveInConstraintExpr(InConstraintExpr, true);
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ilya-biryukov wrote:

Could we rename `InConstraintExpr` to something that matches what we're trying to do better?
The main problem seems to be that it is not just `<constraint-expr>` that may have problems resolving the names of template parameters due to lack of mapping, but also the template arguments that preceded it.

```
<template-args> ::= I <template-arg>* [Q <requires-clause expr>] E
```

Something like `HasIncompleteTemplateParameterTracking` would make the code more readable.

https://github.com/llvm/llvm-project/pull/107385