Reland "Simplify constrained string generation in code generators"

This is a reland of commit 80ad045
CQ and GitHub actions now use newer versions that shouldn't crash
any more when compiling this for release builds.

Original change's description:
> Simplify constrained string generation in code generators
>
> Change-Id: I0ed4abed4a3ef0c7e150971ec58f0aae7e5b0982
> Reviewed-on: https://chrome-internal-review.googlesource.com/c/v8/fuzzilli/+/8591236
> Auto-Submit: Matthias Liedtke <mliedtke@google.com>
> Reviewed-by: Carl Smith <cffsmith@google.com>
> Commit-Queue: Matthias Liedtke <mliedtke@google.com>

Change-Id: I98d5ee50de6057e239d1fae2b0ce4a3dfb1af1bd
Reviewed-on: https://chrome-internal-review.googlesource.com/c/v8/fuzzilli/+/8687236
Reviewed-by: Carl Smith <cffsmith@google.com>
Commit-Queue: Matthias Liedtke <mliedtke@google.com>

Author: Liedtke

Sources/Fuzzilli/Base/ProgramBuilder.swift

@@ -483,6 +483,36 @@ public class ProgramBuilder {
         return probability(0.5) ? randomBuiltinMethodName() : randomCustomMethodName()
     }
 
+    private static func generateConstrained<T: Equatable>(
+            _ generator: () -> T,
+            notIn: any Collection<T>,
+            fallback: () -> T) -> T {
+        var result: T
+        var attempts = 0
+        repeat {
+            if attempts >= 10 {
+                return fallback()
+            }
+            result = generator()
+            attempts += 1
+        } while notIn.contains(result)
+        return result
+    }
+
+    // Generate a string not already contained in `notIn` using the provided `generator`. If it fails
+    // repeatedly, return a random string instead.
+    public func generateString(_ generator: () -> String, notIn: any Collection<String>) -> String {
+        Self.generateConstrained(generator, notIn: notIn,
+            fallback: {String.random(ofLength: Int.random(in: 1...5))})
+    }
+
+    // Find a random variable to use as a string that isn't contained in `notIn`. If it fails
+    // repeatedly, create a random string literal instead.
+    public func findOrGenerateStringLikeVariable(notIn: any Collection<Variable>) -> Variable {
+        return Self.generateConstrained(randomJsVariable, notIn: notIn,
+            fallback: {loadString(String.random(ofLength: Int.random(in: 1...5)))})
+    }
+
     // Settings and constants controlling the behavior of randomParameters() below.
     // This determines how many variables of a given type need to be visible before
     // that type is considered a candidate for a parameter type. For example, if this