Fix the clang driver when "-nostdlib" is present

[sgundapa]

Summary

This patch is a fix to r252901 which changed the behavior of
clang driver. In the presence of "-nostdlib" none of the standard
libraries should be passed to link line.

[dexonsmith]

Testcase?

[sgundapa]

Added a few tests in this new patch set

[vkalintiris]

LGTM. Thank you for working on this.

cfe/trunk/lib/Driver/Tools.cpp

if (!Args.hasArg(options::OPT_nostartfiles)) {
       else
         crtend = isAndroid ? "crtend_android.o" : "crtend.o";
 
       if (HasCRTBeginEndFiles)
         CmdArgs.push_back(Args.MakeArgString(ToolChain.GetFilePath(crtend)));
       if (!isAndroid)
         CmdArgs.push_back(Args.MakeArgString(ToolChain.GetFilePath("crtn.o")));
     }
-  } else if (Args.hasArg(options::OPT_rtlib_EQ))
-    AddRunTimeLibs(ToolChain, D, CmdArgs, Args);
+  }
 
   C.addCommand(llvm::make_unique<Command>(JA, *this, Exec, CmdArgs, Inputs));
 }
 
 // NaCl ARM assembly (inline or standalone) can be written with a set of macros
 // for the various SFI requirements like register masking. The assembly tool
 // inserts the file containing the macros as an input into all the assembly
 // jobs.

cfe/trunk/test/Driver/nostdlib.c

 // RUN: %clang -target i686-pc-linux-gnu -### -nostdlib %s 2> %t
 // RUN: FileCheck < %t %s
 //
 // CHECK-NOT: start-group
 
 // Most of the toolchains would check for -nostartfiles and -nostdlib
 // in a short-circuiting boolean expression, so if both of the preceding
 // options were present, the second would warn about being unused.
 // RUN: %clang -### -nostartfiles -nostdlib -target i386-apple-darwin %s \
 // RUN:   2>&1 | FileCheck %s -check-prefix=ARGSCLAIMED
 // ARGSCLAIMED-NOT: warning:
+
+// In the presence of -nostdlib, the standard libraries should not be
+// passed down to link line
+// RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
+// RUN:     -target i686-pc-linux-gnu -nostdlib --rtlib=compiler-rt \
+// RUN:     -resource-dir=%S/Inputs/resource_dir -lclang_rt.builtins-i686 \
+// RUN:   | FileCheck --check-prefix=CHECK-LINUX-NOSTDLIB %s
+//
+// RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
+// RUN:     -target i686-pc-linux-gnu --rtlib=compiler-rt -nostdlib \
+// RUN:     -resource-dir=%S/Inputs/resource_dir -lclang_rt.builtins-i686 \
+// RUN:   | FileCheck --check-prefix=CHECK-LINUX-NOSTDLIB %s
+//
+// CHECK-LINUX-NOSTDLIB: warning: argument unused during compilation: '--rtlib=compiler-rt'
+// CHECK-LINUX-NOSTDLIB: "{{(.*[^.0-9A-Z_a-z])?}}ld{{(.exe)?}}"
+// CHECK-LINUX-NOSTDLIB-NOT: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}libclang_rt.builtins-i686.a"