[cpu-detection] Cleanup of Host.cpp.
[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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@276149 91177308-0d34-0410-b5e6-96231b3b80d8

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