[Clang][ItaniumMangle] Fix recursive mangling for lambda init-captures (#182667)

[Clang][ItaniumMangle] Fix recursive mangling for lambda init-captures

Mangle computation for lambda signatures can recurse when a call
operator type
references an init-capture (for example via decltype(init-capture)). In
these
cases, mangling can re-enter the init-capture declaration and cycle back
through
operator() mangling.

Make lambda context publication explicit and independent from numbering
state,
then use that context uniformly during mangling:
* Publish lambda `ContextDecl` in `Sema::handleLambdaNumbering()` before
numbering, so dependent type mangling can resolve the lambda context
without
  recursing through the call operator.
* Introduce `CXXRecordDecl::setLambdaContextDecl()` and remove
`ContextDecl`
  from `CXXRecordDecl::LambdaNumbering`.
* Update `ASTImporter` to import/set lambda context separately from
numbering.
* In Itanium mangling, derive init-capture handling from context
computation:
  - map local-lambda init-captures to the enclosing local context in
    `getEffectiveDeclContext()`
  - support init-capture variables in `getClosurePrefix()`
  - keep `mangleLocalName()` generic and rely on the computed context

Add mangling regression coverage in mangle-lambdas.cpp, including:
* local init-captures used through decltype
* non-local variable-template init-captures in decltype
* non-local static inline member init-captures in decltype

* Fixes https://github.com/llvm/llvm-project/issues/63271
* Fixes https://github.com/llvm/llvm-project/issues/86240
* Fixes https://github.com/llvm/llvm-project/issues/139089