[Frontend] Avoid including default system header paths on Fuchsia

[phosek]

Summary

These paths aren't used and don't make sense on Fuchsia.

[mcgrathr]

lgtm

cfe/trunk/lib/Frontend/InitHeaderSearch.cpp

if (HSOpts.UseStandardSystemIncludes) {
     switch (os) {
     case llvm::Triple::CloudABI:
     case llvm::Triple::FreeBSD:
     case llvm::Triple::NetBSD:
     case llvm::Triple::OpenBSD:
     case llvm::Triple::NaCl:
     case llvm::Triple::PS4:
     case llvm::Triple::ELFIAMCU:
+    case llvm::Triple::Fuchsia:
       break;
     case llvm::Triple::Win32:
       if (triple.getEnvironment() != llvm::Triple::Cygnus)
         break;
       LLVM_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: {
     // <isysroot> gets prepended later in AddPath().
     std::string BaseSDKPath = "";
     if (!HasSysroot) {
       const char *envValue = getenv("SCE_ORBIS_SDK_DIR");
       if (envValue)
         BaseSDKPath = envValue;