[ORC] Merge redundant jitlink::Symbol -> JITSymbolFlags mappings.

Adds a getJITSymbolFlagsForSymbol function that returns the JITSymbolFlags
for a given jitlink::Symbol, and replaces severalredundant copies of that
mapping with calls to the new function. This fixes a bug in
LinkGraphMaterializationUnit::scanLinkGraph where we were failing to set the
JITSymbolFlags::Weak flag for weak symbols, and a bug in
ObjectLinkingLayer::claimOrExternalizeWeakAndCommonSymbols where we were
failing to set the JITSymbolFlags::Callable flag for callable symbols.