Bump TSC to latest (0.1.8)

[texasmichelle]

We are currently pulling in a very old version of swift-tools-support-core. This updates to the latest release (0.1.8).

Resolves the following build errors after apple/swift-driver#160.

/swift-base/swift-driver/Sources/SwiftDriver/Utilities/VirtualPath.swift:110:29: error: value of type 'RelativePath' has no member 'appending'
      return .relative(path.appending(component: component))
                       ~~~~ ^~~~~~~~~
/swift-base/swift-driver/Sources/SwiftDriver/Utilities/VirtualPath.swift:112:30: error: value of type 'RelativePath' has no member 'appending'
      return .temporary(path.appending(component: component))
                        ~~~~ ^~~~~~~~~

[texasmichelle]

@swift-ci Please test tensorflow