[Target] Cache the command line derived feature map in TargetOptions.

We can use this to remove some calls to initFeatureMap from Sema
and CodeGen when a function doesn't have a target attribute.

This reduces compile time of the linux kernel where this map
is needed to diagnose some inline assembly constraints based
on whether sse, avx, or avx512 is enabled.

Differential Revision: https://reviews.llvm.org/D85807