[cpu-detection] Cleanup of Host.cpp.

Summary:
Mirroring most cleanup changed from compiler-rt/lib/builtins/cpu_model.
x86 methods are still returning a bool.

Reviewers: llvm-commits, echristo, craig.topper, sanjoy

Subscribers: mehdi_amini

Differential Revision: https://reviews.llvm.org/D22480

llvm-svn: 276149