Instrument.C: do not yield "unknown warning" warnings

gnatcov used to yield "unknown warning" warnings on warning switches not
recognized by clang. Fix this by passing the
"-Wno-unknown-warning-option" when parsing with clang.

Author: Jugst3r

testsuite/tests/instr-cov/144-no-unknown-warning/main.c

@@ -0,0 +1,5 @@
+int
+main (int argc, char **argv)
+{
+  return 0;
+}

testsuite/tests/instr-cov/144-no-unknown-warning/test.py

@@ -0,0 +1,28 @@
+"""
+Check that gnatcov does not yield "unknown warning warnings" when parsing a
+file with warnings not recognized by clang.
+"""
+
+from SCOV.minicheck import build_run_and_coverage, check_xcov_reports
+from SUITE.context import thistest
+from SUITE.cutils import Wdir
+from SUITE.tutils import gprfor
+from SUITE.gprutils import GPRswitches
+
+tmp = Wdir("tmp_")
+
+build_run_and_coverage(
+    gprsw=GPRswitches(
+        root_project=gprfor(
+            srcdirs=[".."], mains=["main.c"], main_cargs=["-Wtrampolines"]
+        )
+    ),
+    covlevel="stmt",
+    mains=["main"],
+    extra_coverage_args=["-axcov", "--output-dir=xcov"],
+    trace_mode="src",
+)
+
+check_xcov_reports("xcov/*.xcov", {"xcov/main.c.xcov": {"+": {4}}})
+
+thistest.result()

tools/gnatcov/instrument-c.adb

@@ -2750,6 +2750,13 @@ package body Instrument.C is
       Append (Command_Line_Args, +"-x");
       Append (Command_Line_Args, +To_Lower (Image (Lang)));
 
+      --  As we also pass compiler warning switches, as they can influence the
+      --  preprocessing through the use of the __has_warning macro, we make
+      --  sure to disable warnings on unknown warnings (as some warnings are
+      --  e.g. gcc-specific and won't be recognized by clang).
+
+      Append (Command_Line_Args, +"-Wno-unknown-warning-option");
+
       return Command_Line_Args;
    end Common_Parse_TU_Args;