Follow up patch for D31495

[yamaguchi]

Summary

This is a follow up patch for https://reviews.llvm.org/D31495.
It broke some build bots, because it had put builtin headers to isystem
prefix.
Eg. when --sysroot=. , it put /home/yamaguchi/test-build/lib/clang/5.0.0/include to ./home/yamaguchi/test-build/lib/clang/5.0.0/include.
I fixed it to check if this is builtin header path or not.

[yamaguchi]

Add comment to code.

clang/lib/Frontend/CompilerInvocation.cpp

for (const Arg *A : Args.filtered(OPT_objcxx_isystem))
     Opts.AddPath(A->getValue(), frontend::ObjCXXSystem, false, true);
 
   // Add the internal paths from a driver that detects standard include paths.
   for (const Arg *A :
        Args.filtered(OPT_internal_isystem, OPT_internal_externc_isystem)) {
     frontend::IncludeDirGroup Group = frontend::System;
     if (A->getOption().matches(OPT_internal_externc_isystem))
       Group = frontend::ExternCSystem;
+
+    // Check if the path is a builtin header or not.
+    // If the path is a builtin header, IgnoreSysRoot is true because otherwise it puts isystem prefix to builtin headers. 
+    SmallString<128> P(Opts.ResourceDir);
+    llvm::sys::path::append(P, "include");
+
+    if (StringRef(A->getValue()) == P)
       Opts.AddPath(A->getValue(), Group, false, true);
+    else
+      Opts.AddPath(A->getValue(), Group, false, false);
   }
 
   // Add the path prefixes which are implicitly treated as being system headers.
   for (const Arg *A :
        Args.filtered(OPT_system_header_prefix, OPT_no_system_header_prefix))
     Opts.AddSystemHeaderPrefix(
         A->getValue(), A->getOption().matches(OPT_system_header_prefix));
 

clang/test/Driver/sysroot-flags.c

 // ISYSROOT_AND_SYSROOT: "--sysroot{{" "|=}}{{[^"]*}}/foo/bar"
 
 // Check that omitting the equals works as well.
 // RUN: %clang -### -target x86_64-unknown-linux-gnu \
 // RUN:   --sysroot /foo/bar -o /dev/null %s 2>&1 | \
 // RUN:   FileCheck %s -check-prefix=SYSROOT_SEPARATE
 // SYSROOT_SEPARATE: "-isysroot" "{{[^"]*}}/foo/bar"
 // SYSROOT_SEPARATE: "--sysroot{{" "|=}}{{[^"]*}}/foo/bar"
+
+// Check that -isysroot is handled properly
+// RUN: %clang -isysroot /foo/bar -c %s -v 2>&1 | \
+// RUN:   grep "/foo/bar"