Replace virtual includes generated paths with their actual paths in the coverage reports.

Fixes #6254.

RELNOTES: The code coverage report now includes the actual paths to header files instead of the ugly,
Bazel generated, virtual includes path.

For example, the `SF:` (source file) lines in the coverage report now look like this:
```
SF:include/common/strategy.h
```
instead of
```
SF:bazel-out/k8-fastbuild/bin/include/common/_virtual_includes/strategy/strategy.h
```

Closes #6372.

PiperOrigin-RevId: 219112911

tools/test/collect_coverage.sh

diff --git a/tools/test/collect_coverage.sh b/tools/test/collect_coverage.sh
index bcdf8fd..a31e083 100755
--- a/tools/test/collect_coverage.sh
+++ b/tools/test/collect_coverage.sh
@@ -103,6 +103,7 @@
     eval "${CC_CODE_COVERAGE_SCRIPT}"
 fi
 
+
 # Export the command line that invokes LcovMerger with the flags:
 # --coverage_dir          The absolute path of the directory where the
 #                         intermediate coverage reports are located.
@@ -124,7 +125,7 @@
 #                         keep only the C++ sources found in the manifest.
 #                         For other languages the sources in the manifest are
 #                         ignored.
-export LCOV_MERGER_CMD="${LCOV_MERGER} --coverage_dir=${COVERAGE_DIR} \
+LCOV_MERGER_CMD="${LCOV_MERGER} --coverage_dir=${COVERAGE_DIR} \
   --output_file=${COVERAGE_OUTPUT_FILE} \
   --filter_sources=/usr/bin/.+ \
   --filter_sources=/usr/lib/.+ \
@@ -132,6 +133,10 @@
   --filter_sources=.*external/.+ \
   --source_file_manifest=${COVERAGE_MANIFEST}"
 
+if [[ $COVERAGE_REPORTED_TO_ACTUAL_SOURCES_FILE ]]; then
+  LCOV_MERGER_CMD="$LCOV_MERGER_CMD\
+  --sources_to_replace_file=$ROOT/$COVERAGE_REPORTED_TO_ACTUAL_SOURCES_FILE"
+fi
 
 if [[ $DISPLAY_LCOV_CMD ]] ; then
   echo "Running lcov_merger"