[Clang] Fix a lambda pattern comparison mismatch after ecc7e6ce4 (#133863)

In ecc7e6ce4, we tried to inspect the `LambdaScopeInfo` on stack to
recover the instantiating lambda captures. However, there was a mismatch
in how we compared the pattern declarations of lambdas: the constraint
instantiation used a tailored `getPatternFunctionDecl()` which is
localized in SemaLambda that finds the very primal template declaration
of a lambda, while `FunctionDecl::getTemplateInstantiationPattern` finds
the latest template pattern of a lambda. This difference causes issues
when lambdas are nested, as we always want the primary template
declaration.

This corrects that by moving `Sema::addInstantiatedCapturesToScope` from
SemaConcept to SemaLambda, allowing it to use the localized version of
`getPatternFunctionDecl`.

It is also worth exploring to coalesce the implementation of
`getPatternFunctionDecl` with
`FunctionDecl::getTemplateInstantiationPattern`. But I’m leaving that
for the future, as I’d like to backport this fix (ecc7e6ce4 made the
issue more visible in clang 20, sorry!), and changing Sema’s ABI would
not be suitable in that regards. Hence, no release note.

Fixes https://github.com/llvm/llvm-project/issues/133719