[JITLink] Silence GCC warnings. NFC.

[mstorsjo]

Summary

Use parentheses in expressions with mixed && and ||. Add unreachable after switch cases.

[mstorsjo]

Tweak the use of unreachable, to avoid warnings when built with clang.

[lhames]

LGTM! Thanks Martin!

llvm/trunk/lib/ExecutionEngine/JITLink/MachOLinkGraphBuilder.h

protected:
   struct NormalizedSymbol {
     friend class MachOLinkGraphBuilder;
 
   private:
     NormalizedSymbol(Optional<StringRef> Name, uint64_t Value, uint8_t Type,
                      uint8_t Sect, uint16_t Desc, Linkage L, Scope S)
         : Name(Name), Value(Value), Type(Type), Sect(Sect), Desc(Desc), L(L),
           S(S) {
-      assert(!Name || !Name->empty() && "Name must be none or non-empty");
+      assert((!Name || !Name->empty()) && "Name must be none or non-empty");
     }
 
   public:
     NormalizedSymbol(const NormalizedSymbol &) = delete;
     NormalizedSymbol &operator=(const NormalizedSymbol &) = delete;
     NormalizedSymbol(NormalizedSymbol &&) = delete;
     NormalizedSymbol &operator=(NormalizedSymbol &&) = delete;