PR12917: Remove incorrect assumption that lambda mangling information cannot

PR12917: Remove incorrect assumption that lambda mangling information cannot
change once it's been assigned. It can change in two ways:
 1) In a template instantiation, the context declaration should be the
    instantiated declaration, not the declaration in the template.
 2) If a lambda appears in the pattern of a variadic pack expansion, the
    mangling number will depend on the pack length.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@160614 91177308-0d34-0410-b5e6-96231b3b80d8