Move ArchSpec to the Utility module

Move ArchSpec to the Utility module

The rationale here is that ArchSpec is used throughout the codebase,
including in places which should not depend on the rest of the code in
the Core module.

This commit touches many files, but most of it is just renaming of
 #include lines. In a couple of cases, I removed the #include ArchSpec
line altogether, as the file was not using it. In one or two places,
this necessitated adding other #includes like lldb-private-defines.h.

llvm-svn: 318048