[lldb] Add support for specifying language when setting watchpoint by expression

This is useful in contexts where you have multiple languages in play:
You may be stopped in a frame for language A, but want to set a watchpoint
with an expression using language B. The current way to do this is to
use the setting `target.language` while setting the watchpoint and
unset it after the watchpoint is set, but that's kind of clunky and
somewhat error-prone. This should add a better way to do this.

rdar://108202559

Differential Revision: https://reviews.llvm.org/D149111