More local mangling fixes.

Compute mangling numbers for externally visible local variables and tags.
Change the mangler to consistently use discriminators where necessary.
Tweak the scheme we use to number decls which are not externally visible
to avoid unnecessary discriminators in common cases now that we request
them more consistently.

Fixes <rdar://problem/14204721>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185986 91177308-0d34-0410-b5e6-96231b3b80d8