[lldb] Tighten lock in Language::ForEach

It is easy to accidentally introduce a deadlock by having the callback
passed to Language::ForEach also attempt to acquire the same lock. It
is easy enough to disallow the callback from calling anything in
Language directly, but it may happen through a series of other
function/method calls.

The solution I am proposing is to tighten the lock in Language::ForEach
so that it is only held as we gather the currently loaded language
plugins. We store them in a vector and then iterate through them with
the callback so that the callback can't introduce a deadlock.

Differential Revision: https://reviews.llvm.org/D109013