Replace ArchSpec::PiecewiseCompare() with Triple::operator==() (#82804)

Looking ast the definition of both functions this is *almost* an NFC
change, except that Triple also looks at the SubArch (important) and
ObjectFormat (less so).

This fixes a bug that only manifests with how Xcode uses the SBAPI to
attach to a process by name: it guesses the architecture based on the
system. If the system is arm64 and the Process is arm64e Target fails to
update the triple because it deemed the two to be equivalent.

rdar://123338218