Rework breakpoint language filtering to use the symbol context's language.

This patch reworks the breakpoint filter-by-language patch to use the
symbol context instead of trying to guess the language solely from the
symbol's name. This has the advantage that symbols compiled with debug
info will have their actual language known. Symbols without debug info
will still do the same "guess"ing because Symbol::GetLanguage() is
implemented using Mangled::GuessLanguage(). The recognition of ObjC
names was merged into Mangled::GuessLanguage.

Reviewed by: jingham, clayborg
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D15326

llvm-svn: 255808