Relax Swift tool check to find swiftc

The Swift-Driver build here only requires a working swiftc, not
necessarily all of SPM. Relaxing the required tool check to accept
building the driver if there's a Swift compiler available.

Author: etcwilde

utils/swift_build_support/swift_build_support/products/earlyswiftdriver.py

@@ -45,7 +45,7 @@ def should_build(self, host_target):
             return False
 
         if self.args.build_early_swift_driver:
-            if toolchain.host_toolchain().find_tool("swift") is None:
+            if toolchain.host_toolchain().find_tool("swiftc") is None:
                 warn_msg = 'Host toolchain could not locate a '\
                            'compiler to build swift-driver. '\
                            '(Try `--skip-early-swift-driver`)'