[ORC] Change handling for SymbolStringPtr tombstones and empty keys.

SymbolStringPtr used to use nullptr as its empty value and (since it performed
ref-count operations on any non-nullptr) a pointer to a special pool-entry
instance as its tombstone.

This commit changes the scheme to use two invalid pointer values as the empty
and tombstone values, and broadens the ref-count guard to prevent ref-counting
operations from being performed on these pointers. This should improve the
performance of SymbolStringPtrs used in DenseMaps/DenseSets, as ref counting
operations will no longer be performed on the tombstone.

llvm-svn: 360925