Currently the code uses FunctionSamples::getCallSiteIdentifier which
will sometimes incorrectly guess that FSAFDO discriminators are probe
based and will convert them incorrectly.
This change doesn't affect builds which don't use FSAFDO, it only fixes
sample profile matching with FS discriminators.
The test for this is manually updated to use discriminator value 15,
which is a perfectly valid base discriminator in the FS world, but
satisfies `isPseudoProbeDiscriminator`, so
`getBaseDiscriminatorFromDiscriminator` will incorrectly extract the
probe index from it.
Note: this change only affects how the base discriminators will be
extracted when doing stale profile matching in the IR-level sample
profile loader. It doesn't add stale profile matching to the MIR-level
FS profile loader pass.
Currently -salvage-stale-profile is a no-op if the profile is not
probe-based. We observed that it can help for regular, non-probe- based
profiles too: some of our internal benchmarks show 0.2-0.3% QPS
improvement.
There seems to be no good reason to limit this flag to only work for
probe-based profiles.
