Revert "[Driver] -m32: Add /usr/include/i386-linux-gnu for Debian"

This reverts commit 82f6e0d which
hasn't addressed the 874bdc8 issue.

Author: petrhosek

clang/lib/Driver/ToolChains/Linux.cpp

@@ -604,8 +604,13 @@ void Linux::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
     return;
   }
 
-  // On Android and Debian, add /usr/include/$triple if exists.
-  std::string MultiarchIncludeDir = getMultiarchTriple(D, getTriple(), SysRoot);
+  // On Android and Debian, add /usr/include/$triple if exists. On Debian, we
+  // can assume a GCC installation is available.
+  std::string MultiarchIncludeDir;
+  if (getTriple().isAndroid())
+    MultiarchIncludeDir = getMultiarchTriple(D, getTriple(), SysRoot);
+  else if (GCCInstallation.isValid())
+    MultiarchIncludeDir = GCCInstallation.getTriple().str();
   if (!MultiarchIncludeDir.empty() &&
       D.getVFS().exists(SysRoot + "/usr/include/" + MultiarchIncludeDir))
     addExternCSystemInclude(DriverArgs, CC1Args, SysRoot + "/usr/include/" + MultiarchIncludeDir);

clang/test/Driver/Inputs/debian_multiarch_tree/usr/include/i386-linux-gnu/.keep

@@ -34,8 +34,6 @@
 // DEBIAN_X86_64_M32-SAME: {{^}} "-internal-isystem" "[[SYSROOT:[^"]+]]/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10/backward"
 // DEBIAN_X86_64_M32-SAME: {{^}} "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 // DEBIAN_X86_64_M32-SAME: {{^}} "-internal-isystem" "[[SYSROOT:[^"]+]]/usr/lib/gcc/x86_64-linux-gnu/10/../../../../x86_64-linux-gnu/include"
-// DEBIAN_X86_64_M32:      "-internal-externc-isystem"
-// DEBIAN_X86_64_M32-SAME: {{^}} "[[SYSROOT]]/usr/include/i386-linux-gnu"
 // DEBIAN_X86_64_M32:      "-L
 // DEBIAN_X86_64_M32-SAME: {{^}}[[SYSROOT]]/usr/lib/gcc/x86_64-linux-gnu/10/32"
 // DEBIAN_X86_64_M32-SAME: {{^}} "-L[[SYSROOT]]/usr/lib/gcc/x86_64-linux-gnu/10/../../../../lib32"