Now that the cycle between MCTargetDesc and TargetInfo is gone
(see revisions 360709 360718 360722 360724 360726 360731 360733 360735 360736),
remove the dependency from TargetInfo on MCTargetDesc:tablegen. In most
targets, this makes MCTargetDesc:tablegen have just a single use, so
inline it there.
For AArch64, ARM, and RISCV there's still a similar cycle between
MCTargetDesc and Utils, so the MCTargetDesc:tablegen indirection is
still needed there.