[NFC] isOSGlibc: musl is not glibc. (#171734)
Previously, `isOSGlibc()` was returning true for musl triples as well.
This commit changes `isOSGlibc()` to return false for musl triples, and
updates all existing `isOSGlibc()` checks to call `isOSGlibc() ||
isMusl()`, in order to preserve existing behaviour.
