Internal coverage: Backout -fkeep-inline-functions.

nodist/code_coverage

@@ -41,7 +41,13 @@ def test():
         cxx_flags = (
             "--coverage"
             # Otherwise inline may not show as uncovered
-            + " -fkeep-inline-functions"
+            #  If we use this then e.g. verilated.h functions properly show up
+            #  if unused.
+            #  However, VerilatedSerialize::write then changes from covered
+            #  to uncovered (in G++ 9.3.0) even with all inlining turned off.
+            #  Having false negative coverage is more effort then missing negatives.
+            #  Also this seems to explode the runtime (since a lot more data).
+            #+ " -fkeep-inline-functions"
             # Otherwise static may not show as uncovered
             + " -fkeep-static-functions"
             # Exceptions can pollute the branch coverage data
@@ -135,6 +141,8 @@ def test():
         run(RealPath + "/fastcov.py -b -c src/obj_dbg -X --lcov" +
             #            " --exclude /usr --exclude test_regress" + " -o " + cc_dir +
             " -o " + cc_dir + "/app_total.info")
+        # For debug to convert single .gcna/.gcno in a directory to cov.info:
+        #    lcov -c -d . -o cov.info
         ci_fold_end()
 
     if Args.stage_enabled[6]: