[BOLT] Improve handling of secondary function entry points

Summary:
"Fix symbol table entries for secondary entries" diff broke the inliner.

Fix the breakage and make the discovery of secondary entry points more
accurate.

Add ability to BinaryContext::getFunctionForSymbol() to return an entry
point discriminator and use it instead of calling getEntryForSymbol()
and isSecondaryEntry(). This is the preferred way since
getFunctionForSymbol() is thread-safe.

(cherry picked from FBD19295983)