BaseTools/Plugin/HostBasedUnitTestRunner: Merge clang coverage files

For clang build, enable to generate coverage file under Build folder to
merge coverage files of all module.

Signed-off-by: Cosmo Lai <cosmo.lai@intel.com>

Author: cosmolai

BaseTools/Plugin/HostBasedUnitTestRunner/HostBasedUnitTestRunner.py

@@ -272,13 +272,25 @@ def gen_code_coverage_clang(self, thebuilder):
             if ret != 0:
                 logging.error("UnitTest Coverage: Failed generate filtered coverage XML.")
                 return 1
-        # Merge all XML files
+        # Merge package coverage XML files
         ret = self.merge_cobertura_xml_files([f"{testFile}.coverage.xml" for testFile in testList], mergedCoverageXml)
+        if ret != 0:
+            logging.error("UnitTest Coverage: Failed to merge package coverage XML files.")
+            return 1
+
+        workspace = thebuilder.env.GetValue("WORKSPACE")
+        allCoverageXml = os.path.join(workspace, 'Build', 'coverage.xml')
+        testCoverageList = glob.glob(os.path.join(workspace, 'Build', "**", "*.coverage.xml"), recursive=True)
+
+        # Merge all coverage XML files
+        ret = self.merge_cobertura_xml_files(testCoverageList, allCoverageXml)
         if ret != 0:
             logging.error("UnitTest Coverage: Failed to merge coverage XML files.")
             return 1
+
         return 0
 
+
     def merge_cobertura_xml_files(self, xml_file_list, output_file):
         """
         Merge multiple Cobertura XML files into a single output file.
@@ -394,7 +406,7 @@ def gen_code_coverage_msvc(self, thebuilder):
                 logging.error("UnitTest Coverage: Failed to collect coverage data.")
                 return 1
 
-        # Generate and XML file if requested.by each package
+        # Generate and XML file if requested by each package
         ret = RunCmd(
             "OpenCppCoverage",
             f"--export_type cobertura:{os.path.join(buildOutputBase, 'coverage.xml')} " +