[lldb] Remove FileSpec->CompileUnit inheritance

Summary:
CompileUnit is a complicated class. Having it be implicitly convertible
to a FileSpec makes reasoning about it even harder.

This patch replaces the inheritance by a simple member and an accessor
function. This avoid the need for casting in places where one needed to
force a CompileUnit to be treated as a FileSpec, and does not add much
verbosity elsewhere.

It also fixes a bug where we were wrongly comparing CompileUnit& and a
CompileUnit*, which compiled due to a combination of this inheritance
and the FileSpec*->FileSpec implicit constructor.

Reviewers: teemperor, JDevlieghere, jdoerfert

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D70827