Don't skip lambdas when mangling local vars.

This commit rearranges the logic in CXXNameMangler::mangleLocalName and
GetLocalClassDecl so that it doesn't accidentally skip over lambdas.  It
also reduces code duplication a bit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185402 91177308-0d34-0410-b5e6-96231b3b80d8