[5.5] Switch branch dependencies on swift-tools-support-core, swift-llbuild, and swift-driver from main to release/5.5 (as usual for every release). (#3465)

Author: abertelrud

Package.swift

@@ -335,7 +335,7 @@ let package = Package(
 // this right now.
 
 /// When not using local dependencies, the branch to use for llbuild and TSC repositories.
-let relatedDependenciesBranch = "main"
+let relatedDependenciesBranch = "release/5.5"
 
 if ProcessInfo.processInfo.environment["SWIFTPM_LLBUILD_FWK"] == nil {
     if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {