Don't use sysroot/include when sysroot is empty.

[abidh]

Summary

Baremetal toolchain add Driver.SysRoot/include to the system include
paths without checking if Driver.SysRoot is empty. This resulted in
"-internal-isystem" "include" in the command. This patch adds check for
empty sysroot.

[manojgupta]

lgtm but not an expert in this area.

[cjdb]

In D92176#2419011, @manojgupta wrote:

lgtm but not an expert in this area.

Ditto

[abidh]

@jroelofs Do you have any comments on this patch?

[jroelofs]

LGTM

clang/lib/Driver/ToolChains/BareMetal.cpp

 
   if (!DriverArgs.hasArg(options::OPT_nostdlibinc)) {
     SmallString<128> Dir(getDriver().SysRoot);
-    llvm::sys::path::append(Dir, "include");
-    addSystemInclude(DriverArgs, CC1Args, Dir.str());
+    if (!Dir.empty()) {
+      llvm::sys::path::append(Dir, "include");
+      addSystemInclude(DriverArgs, CC1Args, Dir.str());
+    }
   }
 }
 

clang/test/Driver/baremetal.cpp

 // RUN:   | FileCheck %s --check-prefix=CHECK-RTLIB-GCC
 // CHECK-RTLIB-GCC: -lgcc
 
+// RUN: %clang -### -target arm-none-eabi -v %s 2>&1 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-SYSROOT-INC
+// CHECK-SYSROOT-INC-NOT: "-internal-isystem" "include"
+
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target riscv64-unknown-elf \
 // RUN:     -L some/directory/user/asked/for \