[lldb][test] Add --sysroot argument to dotest.py

This argument allows to set specific sysroot pass which will be used for
building LLDB API test programs.
It might come in handy for setting up cross-platform remote runs of API
tests on Windows host.

It can be useful for cross-compiling LLDB API tests. The argument can be
set using `LLDB_TEST_USER_ARGS` argument:
```
cmake ...
-DLLDB_TEST_USER_ARGS="...;--sysroot;C:\path\to\sysroot;..."
...
```

Author: dzhidzhoev

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

@@ -304,7 +304,9 @@ def parseOptionsAndInitTestdirs():
         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:
+    if args.sysroot is not None:
+        configuration.sdkroot = args.sysroot
+    elif 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)
         )

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

@@ -49,6 +49,15 @@ def create_parser():
             """Specify the compiler(s) used to build the inferior executables. The compiler path can be an executable basename or a full path to a compiler executable. This option can be specified multiple times."""
         ),
     )
+    group.add_argument(
+        "--sysroot",
+        metavar="sysroot",
+        dest="sysroot",
+        default="",
+        help=textwrap.dedent(
+            """Specify the path to sysroot. This overrides apple_sdk sysroot."""
+        ),
+    )
     if sys.platform == "darwin":
         group.add_argument(
             "--apple-sdk",