Lots of code around LLDB was directly accessing the target's section
load list. This NFC patch makes the section load list private so the
Target class can access it, but everyone else now uses accessor
functions. This allows us to control the resolving of addresses and will
allow for functionality in LLDB which can lazily resolve addresses in
JIT plug-ins with a future patch.

This commit does a few related things:
- Removes unused function `uuid_is_null`
- Removes unneeded includes of UuidCompatibility.h
- Renames UuidCompatibility to AppleUuidCompatibility and adds a comment
  to clarify intent of header.
- Moves AppleUuidCompatibility to the include directory

Differential Revision: https://reviews.llvm.org/D156562

There's no reason for FileSpecList to live in lldb/Core while FileSpec
lives in lldb/Utility. Move FileSpecList next to FileSpec.

In the spirit of not having lldbExpression rely on plugins, this move
makes the most sense. ObjectFileJIT is not really a "plugin" in the
sense that without it, expression evaluation doesn't work at all. This
is different than something like ObjectFileELF where lldb can still
technically debug non-ELF targets without it. For that reason, moving
ObjectFileJIT into Expression where it will be used in conjunction with
IRExecutionUnit makes the most sense.

Differential Revision: https://reviews.llvm.org/D147084