[clang] Construct ExprRequirement with SubstitutionDiagnostic on SubstFailure

We're expecting a SubstitutionDiagnostic in diagnoseUnsatisfiedRequirement
if the status of ExprRequirement is SubstFailure. Previously, the Requirement
was created with Expr on SubstFailure by mistake, which could lead to the
assertion failure in the subsequent diagnosis.

Fixes https://github.com/clangd/clangd/issues/1726
Fixes https://github.com/llvm/llvm-project/issues/64723
Fixes https://github.com/llvm/llvm-project/issues/64172

In addition, this patch also fixes an invalid test from D129499.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D158061