[lldb] Don't let dotest silently fall back to default SDK if the specified one can't be found

[teemperor]

Summary

If the SDK name passed to dotest can't be found by xcrun we silently fall back to the default SDK.
This leads to rather cryptic errors being reported later on when linking the actual test executables.

Instead just directly log and abort when this situation is encountered and inform the user about
the invalid argument.

lldb/packages/Python/lldbsuite/test/dotest.py

def parseOptionsAndInitTestdirs():
     if args.out_of_tree_debugserver:
         lldbtest_config.out_of_tree_debugserver = args.out_of_tree_debugserver
 
     # Set SDKROOT if we are using an Apple SDK
     if platform_system == 'Darwin' and args.apple_sdk:
         configuration.sdkroot = seven.get_command_output(
             'xcrun --sdk "%s" --show-sdk-path 2> /dev/null' %
             (args.apple_sdk))
+        if not configuration.sdkroot:
+            logging.error(
+                    'No SDK found with the name %s; aborting...',
+                    args.apple_sdk)
+            sys.exit(-1)
 
     if args.arch:
         configuration.arch = args.arch
     else:
         configuration.arch = platform_machine
 
     if args.categories_list:
         configuration.categories_list = set(