[X86] Move frontend CPU feature initialization to a look up table based implementation. NFCI

This replaces the switch statement implementation in the clang's
X86.cpp with a lookup table in X86TargetParser.cpp.

I've used constexpr and copy of the FeatureBitset from
SubtargetFeature.h to store the features in a lookup table.
After the lookup the bitset is translated into strings for use
by the rest of the frontend code.

I had to modify the implementation of the FeatureBitset to avoid
bugs in gcc 5.5 constexpr handling. It seems to not like the
same array entry to be used on the left side and right hand side
of an assignment or &= or |=. I've also used uint32_t instead of
uint64_t and sized based on the X86::CPU_FEATURE_MAX.

I've initialized the features for different CPUs outside of the
table so that we can express inheritance in an adhoc way. This
was one of the big limitations of the switch and we had resorted
to labels and gotos.

Differential Revision: https://reviews.llvm.org/D82731

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