[CIR] Disable -fclangir-call-conv-lowering from default in the LLVM pipeline

This is causing lots of churn. `-fclangir-call-conv-lowering` is not mature
enough, assumptions are leading to crashes we cannot track with special
messages, leading to not great user experience. Turn this off until we have
someone dedicated to roll this out.

Author: bcardosolopes

clang/include/clang/Driver/Options.td

@@ -3059,7 +3059,7 @@ defm clangir_analysis_only : BoolFOption<"clangir-analysis-only",
 // FIXME(cir): Remove this option once all pre-existing tests are compatible with
 // the calling convention lowering pass.
 defm clangir_call_conv_lowering : BoolFOption<"clangir-call-conv-lowering",
-  FrontendOpts<"ClangIRCallConvLowering">, DefaultTrue,
+  FrontendOpts<"ClangIRCallConvLowering">, DefaultFalse,
   PosFlag<SetTrue, [], [ClangOption, CC1Option], "Transform CIR to abide to calling convetions during lowering">,
   NegFlag<SetFalse, [], [ClangOption, CC1Option], "Ignore calling convetion during lowering">,
   BothFlags<[], [ClangOption, CC1Option], "">>;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5228,6 +5228,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
   Args.addOptOutFlag(CmdArgs, options::OPT_fclangir_call_conv_lowering,
                      options::OPT_fno_clangir_call_conv_lowering);
+  Args.addOptInFlag(CmdArgs, options::OPT_fclangir_call_conv_lowering,
+                    options::OPT_fno_clangir_call_conv_lowering);
   if (Args.hasArg(options::OPT_fclangir_mem2reg))
     CmdArgs.push_back("-fclangir-mem2reg");
 

clang/test/CIR/Driver/callconv.cpp

@@ -1,4 +1,4 @@
 // RUN: %clang %s -fno-clangir-call-conv-lowering -### -c %s 2>&1 | FileCheck --check-prefix=DISABLE %s
 // DISABLE: "-fno-clangir-call-conv-lowering"
 // RUN: %clang %s -fclangir-call-conv-lowering -### -c %s 2>&1 | FileCheck --check-prefix=ENABLE %s
-// ENABLE-NOT: "-fclangir-call-conv-lowering"
+// ENABLE: "-fclangir-call-conv-lowering"