__builtin_cpu_supports should be converted to a constant based on -march

__builtin_cpu_supports(“foo”) should be collapsed to a constant if foo is unconditionally enabled by -march, for example __builtin_cpu_supports(“avx2”) should be true with -mavx2.

This will greatly simplify use cases that support both static and runtime dispatch.

Start by locating the implementation of __builtin_cpu_supports and the handling of -march in LLVM. Trace how enabled CPU features are represented and how the builtin is lowered, then identify the relevant compiler tests. Done means the builtin becomes a compile-time constant when the requested feature is unconditionally enabled by -march, while runtime dispatch remains supported otherwise.

Written by the indexing model from the issue text.