[Darwin][Driver][clang] apple-none-macho orders the resource directory after internal-externc-isystem when nostdlibinc is used (#122035)

Embedded development often needs to use a different C standard library,
replacing the existing one normally passed as -internal-externc-isystem.
This works fine for an apple-macos target, but apple-none-macho doesn't
work because the MachO driver doesn't implement
AddClangSystemIncludeArgs to add the resource directory as
-internal-isystem like most other drivers do. Move most of the search
path logic from Darwin and DarwinClang down into an AppleMachO toolchain
between the MachO and Darwin toolchains.

Also define __MACH__ for apple-none-macho, as Swift expects all MachO
targets to have that defined.

1 files changed, 5 insertions, 0 deletions

diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index 29723b5..8eba766 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -1507,6 +1507,11 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
   // ELF targets define __ELF__
   if (TI.getTriple().isOSBinFormatELF())
     Builder.defineMacro("__ELF__");
+  else if (TI.getTriple().isAppleMachO())
+    // Apple MachO targets define __MACH__ even when not using DarwinTargetInfo.
+    // Hurd will also define this in some circumstances, but that's done in
+    // HurdTargetInfo. Windows targets don't define this.
+    Builder.defineMacro("__MACH__");
 
   // Target OS macro definitions.
   if (PPOpts.DefineTargetOSMacros) {