clang-tidy: Fix explicit --driver-mode= argument

Since commit f6f4eb0907 (clang-tidy: Add driver mode argument,
2020-01-19) the `clang-tidy` tool rejects the new option:

    Unknown command line argument '--driver-mode=g++'.

This is because we are adding the flag before the main compiler
command-line.  Encode it with `--extra-arg-before=` so that `clang-tidy`
knows it is supposed to be part of the compiler command.

Suggested-by: Hanjiang Yu

2 files changed, 2 insertions, 2 deletions

diff --git a/Source/cmMakefileTargetGenerator.cxx b/Source/cmMakefileTargetGenerator.cxx
index a2fa3b1..d7e2de6 100644
--- a/Source/cmMakefileTargetGenerator.cxx
+++ b/Source/cmMakefileTargetGenerator.cxx
@@ -788,7 +788,7 @@ void cmMakefileTargetGenerator::WriteObjectRuleFiles(
             driverMode = lang == "C" ? "gcc" : "g++";
           }
           run_iwyu += this->LocalGenerator->EscapeForShell(
-            cmStrCat(tidy, ";--driver-mode=", driverMode));
+            cmStrCat(tidy, ";--extra-arg-before=--driver-mode=", driverMode));
         }
         if (cpplint && *cpplint) {
           run_iwyu += " --cpplint=";
diff --git a/Source/cmNinjaTargetGenerator.cxx b/Source/cmNinjaTargetGenerator.cxx
index 5e8cc7d..0e74678 100644
--- a/Source/cmNinjaTargetGenerator.cxx
+++ b/Source/cmNinjaTargetGenerator.cxx
@@ -767,7 +767,7 @@ void cmNinjaTargetGenerator::WriteCompileRule(const std::string& lang,
           driverMode = lang == "C" ? "gcc" : "g++";
         }
         run_iwyu += this->GetLocalGenerator()->EscapeForShell(
-          cmStrCat(tidy, ";--driver-mode=", driverMode));
+          cmStrCat(tidy, ";--extra-arg-before=--driver-mode=", driverMode));
       }
       if (cpplint && *cpplint) {
         run_iwyu += " --cpplint=";