[Driver] move ELFIAMCU / RTEMS header search path management to the driver

[brad]

Summary

As has been done for other OS's. Move the header path management to the driver.

[brad]

Rebase.

clang/lib/Lex/InitHeaderSearch.cpp

void InitHeaderSearch::AddDefaultCIncludePaths(const llvm::Triple &triple,
                                             const HeaderSearchOptions &HSOpts) {
   if (!ShouldAddDefaultIncludePaths(triple))
     llvm_unreachable("Include management is handled in the driver.");
 
   llvm::Triple::OSType os = triple.getOS();
 
   if (HSOpts.UseStandardSystemIncludes) {
     switch (os) {
-    case llvm::Triple::ELFIAMCU:
-      break;
     case llvm::Triple::Win32:
       if (triple.getEnvironment() != llvm::Triple::Cygnus)
         break;
       [[fallthrough]];
     default:
       // FIXME: temporary hack: hard-coded paths.
       AddPath("/usr/local/include", System, false);
       break;
if (CIncludeDirs != "") {
     SmallVector<StringRef, 5> dirs;
     CIncludeDirs.split(dirs, ":");
     for (StringRef dir : dirs)
       AddPath(dir, ExternCSystem, false);
     return;
   }
 
   switch (os) {
-  case llvm::Triple::RTEMS:
-    break;
   case llvm::Triple::Win32:
     switch (triple.getEnvironment()) {
     default: llvm_unreachable("Include management is handled in the driver.");
     case llvm::Triple::Cygnus:
       AddPath("/usr/include/w32api", System, false);
       break;
     case llvm::Triple::GNU:
       break;
     }
     break;
   default:
     break;
   }
 
-  switch (os) {
-  case llvm::Triple::RTEMS:
-  case llvm::Triple::ELFIAMCU:
-    break;
-  default:
-    AddPath("/usr/include", ExternCSystem, false);
-    break;
-  }
 }
 
 void InitHeaderSearch::AddDefaultCPlusPlusIncludePaths(
     const LangOptions &LangOpts, const llvm::Triple &triple,
     const HeaderSearchOptions &HSOpts) {
   if (!ShouldAddDefaultIncludePaths(triple))
     llvm_unreachable("Include management is handled in the driver.");
 
default:
     break;
   }
 }
 
 bool InitHeaderSearch::ShouldAddDefaultIncludePaths(
     const llvm::Triple &triple) {
   switch (triple.getOS()) {
   case llvm::Triple::AIX:
+  case llvm::Triple::ELFIAMCU:
   case llvm::Triple::Emscripten:
   case llvm::Triple::FreeBSD:
   case llvm::Triple::Fuchsia:
   case llvm::Triple::Haiku:
   case llvm::Triple::Hurd:
   case llvm::Triple::Linux:
   case llvm::Triple::LiteOS:
   case llvm::Triple::NaCl:
   case llvm::Triple::NetBSD:
   case llvm::Triple::OpenBSD:
   case llvm::Triple::PS4:
   case llvm::Triple::PS5:
+  case llvm::Triple::RTEMS:
   case llvm::Triple::Solaris:
   case llvm::Triple::WASI:
   case llvm::Triple::ZOS:
     return false;
 
   case llvm::Triple::Win32:
     if (triple.getEnvironment() != llvm::Triple::Cygnus ||
         triple.isOSBinFormatMachO())