[bootstrap] Improve CMake caching
[bootstrap] Improve CMake caching

Previously, we would only care if "CMakeCache.txt" exists, this adds a
check for whether we are using the correct `swiftc`. This resolves and
issue when switching between different versions of Xcode.

rdar://problem/46454067

apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager