Don't create a SymbolTable in Function when the LLVMContext discards value names (NFC)

The ValueSymbolTable is used to detect name conflict and rename
instructions automatically. This is not needed when the value
names are automatically discarded by the LLVMContext.
No functional change intended, just saving a little bit of memory.

This is a recommit of r281806 after fixing the accessor to return
a pointer instead of a reference and updating all the call-sites.

llvm-svn: 281813