[Driver] move Fuchsia header search path management to the driver

[brad]

Summary

Fuchsia already implements AddClangSystemIncludeArgs(). So it looks like we just have to switch over to using it.

[phosek]

LGTM

clang/lib/Lex/InitHeaderSearch.cpp

void InitHeaderSearch::AddDefaultCIncludePaths(const llvm::Triple &triple,
 
   if (HSOpts.UseStandardSystemIncludes) {
     switch (os) {
     case llvm::Triple::CloudABI:
     case llvm::Triple::NaCl:
     case llvm::Triple::PS4:
     case llvm::Triple::PS5:
     case llvm::Triple::ELFIAMCU:
-    case llvm::Triple::Fuchsia:
       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);
default:
     break;
   }
 
   switch (os) {
   case llvm::Triple::CloudABI:
   case llvm::Triple::RTEMS:
   case llvm::Triple::NaCl:
   case llvm::Triple::ELFIAMCU:
-  case llvm::Triple::Fuchsia:
     break;
   case llvm::Triple::PS4:
   case llvm::Triple::PS5: {
     // <isysroot> gets prepended later in AddPath().
     std::string BaseSDKPath;
     if (!HasSysroot) {
       const char *EnvVar = (os == llvm::Triple::PS4) ? "SCE_ORBIS_SDK_DIR"
                                                      : "SCE_PROSPERO_SDK_DIR";

 bool InitHeaderSearch::ShouldAddDefaultIncludePaths(
     const llvm::Triple &triple) {
   switch (triple.getOS()) {
   case llvm::Triple::AIX:
   case llvm::Triple::Emscripten:
   case llvm::Triple::FreeBSD:
   case llvm::Triple::NetBSD:
   case llvm::Triple::OpenBSD:
+  case llvm::Triple::Fuchsia:
   case llvm::Triple::Hurd:
   case llvm::Triple::Linux:
   case llvm::Triple::Solaris:
   case llvm::Triple::WASI:
     return false;
 
   case llvm::Triple::Win32:
     if (triple.getEnvironment() != llvm::Triple::Cygnus ||