[Target] Remove Process::GetCPPLanguageRuntime

Summary:
I want to remove this method because I think that Process should be
language agnostic, or at least, not have knowledge about specific language
runtimes. There is "GetLanguageRuntime()" which should be used instead. If the
caller a CPPLanguageRuntime, they should cast it as needed. Ideally, this
should only happen in plugins that need C++ specific knowledge.

The next step I would like to do is remove "GetObjCLanguageRuntime()" as well.
There are a lot more instances of that function being used, so I wanted to
upload this one first to get the general reception to this idea.

Reviewers: compnerd, davide, JDevlieghere, jingham, clayborg, labath, aprantl

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D62755

llvm-svn: 362544