[BOLT] Make Relocations a class and add optional field (#131638)

This patch converts `Relocations` from a struct to a class, and
introduces the `Optional` field. Patch #116964 will use it.

Some optimizations, like `scanExternalRefs`, create relocations that
patch the old code. Under certain circumstances these may be skipped
without correctness implications.

bolt/include/bolt/Core/Relocation.h

@@ -35,7 +35,16 @@ enum { R_X86_64_converted_reloc_bit = 0x80 };
 namespace bolt {
 
 /// Relocation class.
-struct Relocation {
+class Relocation {
+public:
+  Relocation(uint64_t Offset, MCSymbol *Symbol, uint32_t Type, uint64_t Addend,
+             uint64_t Value)
+      : Offset(Offset), Symbol(Symbol), Type(Type), Optional(false),
+        Addend(Addend), Value(Value) {}
+
+  Relocation()
+      : Offset(0), Symbol(0), Type(0), Optional(0), Addend(0), Value(0) {}
+
   static Triple::ArchType Arch; /// set by BinaryContext ctor.
 
   /// The offset of this relocation in the object it is contained in.
@@ -47,6 +56,11 @@ struct Relocation {
   /// Relocation type.
   uint32_t Type;
 
+private:
+  /// Relocations added by optimizations can be optional.
+  bool Optional = false;
+
+public:
   /// The offset from the \p Symbol base used to compute the final
   /// value of this relocation.
   uint64_t Addend;
@@ -58,6 +72,10 @@ struct Relocation {
   /// Return size in bytes of the given relocation \p Type.
   static size_t getSizeForType(uint32_t Type);
 
+  /// Some relocations added by optimizations are optional, meaning they can be
+  /// omitted under certain circumstances.
+  void setOptional() { Optional = true; }
+
   /// Return size of this relocation.
   size_t getSize() const { return getSizeForType(Type); }