[clang] Requesting a public API for the substituted-without-satisfaction form of a constraint expression

I'd kindly ask to expose a supported way for obtaining the template-argument-substituted form of a constraint expression.

**Motivation**: MrDocs (a Clang-based C++ documentation generator) computes a stable symbol identity for constrained templates: two templates differing only in their requires-clause must get distinct, stable IDs across redeclarations. We do that by hashing (`ODRHash`) the _substituted_/_normalized_ constraint expression. AFAICS, the only code that currently yields that expression is the `static` `SubstituteConstraintExpressionWithoutSatisfaction`. The public entry point that uses it, `Sema::AreConstraintExpressionsEqual`, just returns a `bool` and doesn't hand back the substituted expression. So, we vendor a copy, which reaches into `Sema` internals and has to be re-synced by hand on every LLVM update.

For our purposes, it would be enough if `SubstituteConstraintExpressionWithoutSatisfaction` were promoted to a public `Sema` function.

**Version**: 77e43ec1.

Start by reading the static SubstituteConstraintExpressionWithoutSatisfaction implementation and the public Sema::AreConstraintExpressionsEqual entry point to understand their current interfaces and constraints. Done means Clang exposes a supported public Sema way to obtain the substituted constraint expression without satisfaction, allowing MrDocs to stop vendoring this logic.

Written by the indexing model from the issue text.