[Target] Decouple ObjCLanguageRuntime from LanguageRuntime

Summary:
ObjCLanguageRuntime was being pulled into LanguageRuntime because of
Breakpoint Preconditions. If we move BreakpointPrecondition out of Breakpoint,
we can extend the LanguageRuntime plugin interface so that LanguageRuntimes
can give us a BreakpointPrecondition for exceptions.

Differential Revision: https://reviews.llvm.org/D63181

llvm-svn: 364098