[lld] [LLD] Improve linker script handing in LLD (PR #106334)

[Hongyu Chen via llvm-commits]

================
@@ -74,6 +75,106 @@ struct ExprValue {
 // Later, we evaluate the expression by calling the function.
 using Expr = std::function<ExprValue()>;
 
+class ScriptExpr {
+public:
+  enum class ExprKind : uint8_t { Constant, Dynamic, Unary, Binary };
+
+private:
+  ExprKind kind_;
+
+protected:
+  explicit ScriptExpr(ExprKind kind) : kind_(kind) {}
+
+public:
+  ExprKind getKind() const { return kind_; }
+  std::function<ExprValue()> getExpr() const;
+  ExprValue getExprValue() const;
+};
+
+class ConstantExpr : public ScriptExpr {
+public:
+  ConstantExpr(uint64_t val) : ScriptExpr(ExprKind::Constant), val_(val) {}
+  ExprValue getExprValue() const { return ExprValue(val_); }
+
+private:
+  uint64_t val_;
+};
+
+class DynamicExpr : public ScriptExpr {
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yugier wrote:

I agree that the name `DynamicExpr` is confusing since dynamic has a special meaning in the linker while what we have here has nothing to do with dynamic. I changed the name to `FallbackExpr` since we are temporarily using `FallbackExpr` to carry the lambdas that we cannot represent as `BinaryExpr`, `UnaryExpr`, and `ConstantExpr`. Yes potentially we can add more sub-classes to completely replace all lambda functions but we are still trying to figure  out how to separate information and function calls in the lambda functions; `FallbackExpr` is more like a transition state for eventually removing all lambdas functions we have. 

Hopefully I describe the motivation for `DynamicExpr` or `FallbackExpr` clearly here and please let me know if anything I can elaborate here. Any feedback or suggestions on extracting information we need in lambda functions out so we can remove them would be appreciated! 

https://github.com/llvm/llvm-project/pull/106334