[clang-format] use spaces for alignment with UT_ForContinuationAndIndentation

Summary:
Use spaces instead of tabs for alignment with UT_ForContinuationAndIndentation to make the code aligned for any tab/indent width.

Fixes https://bugs.llvm.org/show_bug.cgi?id=38381

Reviewed By: MyDeveloperDay

Patch By: fickert

Tags: #clang-format

Differential Revision: https://reviews.llvm.org/D75034