[llvm-cov] Enable -show-directory-coverage for LLVM itself.

My previous patch D151283 added a new option to llvm-cov to make it generate a hierarchical report for better view. This patch is to enable it for the coverage report of LLVM itself.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D158703

llvm/utils/prepare-code-coverage-artifact.py

@@ -67,6 +67,7 @@ def prepare_html_report(
             "-o",
             report_dir,
             "-show-line-counts-or-regions",
+            "-show-directory-coverage",
             "-Xdemangler",
             "c++filt",
             "-Xdemangler",