Replace HostInfo::GetLLDBPath with specific functions

Summary:
Instead of a function taking an enum value determining which path to
return, we now have a suite of functions, each returning a single path
kind. This makes it easy to move the python-path function into a
specific plugin in a follow-up commit.

All the users of GetLLDBPath were converted to call specific functions
instead. Most of them were hard-coding the enum value anyway, so this
conversion was simple. The only exception was SBHostOS, which I've
changed to use a switch on the incoming enum value.

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D48272

llvm-svn: 335052