[Driver] Allow drivers to add multiple libc++ include paths

This allows toolchain drivers to add multiple libc++ include paths akin
to libstdc++. This is useful in multiarch setup when some headers might
be in target specific include directory. There should be no functional
change.

Differential Revision: https://reviews.llvm.org/D45422

llvm-svn: 329748
diff --git a/clang/lib/Driver/ToolChains/MipsLinux.cpp b/clang/lib/Driver/ToolChains/MipsLinux.cpp
index b394208..2f1c8b2 100644
--- a/clang/lib/Driver/ToolChains/MipsLinux.cpp
+++ b/clang/lib/Driver/ToolChains/MipsLinux.cpp
@@ -94,16 +94,18 @@
   return ToolChain::CST_Libcxx;
 }
 
-std::string MipsLLVMToolChain::findLibCxxIncludePath() const {
+void MipsLLVMToolChain::addLibCxxIncludePaths(
+    const llvm::opt::ArgList &DriverArgs,
+    llvm::opt::ArgStringList &CC1Args) const {
   if (const auto &Callback = Multilibs.includeDirsCallback()) {
     for (std::string Path : Callback(SelectedMultilib)) {
       Path = getDriver().getInstalledDir() + Path + "/c++/v1";
       if (llvm::sys::fs::exists(Path)) {
-        return Path;
+        addSystemInclude(DriverArgs, CC1Args, Path);
+        return;
       }
     }
   }
-  return "";
 }
 
 void MipsLLVMToolChain::AddCXXStdlibLibArgs(const ArgList &Args,