This moves the registry higher in the LLVM library dependency stack.
Every client of the target registry needs to link against MC anyway to
actually use the target, so we might as well move this out of Support.
This allows us to ensure that Support doesn't have includes from MC/*.
Differential Revision: https://reviews.llvm.org/D111454
At the moment `getMCInstrBeads` is forward-declared in a few places,
bring this together into a single header file.
This was done as part of the disassembler work, since the disassembler
would otherwise add one more forward declaration.
Differential Revision: https://reviews.llvm.org/D98533
