Add default initializer for Sysroot ivar.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@86610 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/clang/Frontend/HeaderSearchOptions.h b/include/clang/Frontend/HeaderSearchOptions.h
index dc222c1..c0bd1a9 100644
--- a/include/clang/Frontend/HeaderSearchOptions.h
+++ b/include/clang/Frontend/HeaderSearchOptions.h
@@ -66,7 +66,7 @@
   unsigned Verbose : 1;
 
 public:
-  HeaderSearchOptions(llvm::StringRef _Sysroot)
+  HeaderSearchOptions(llvm::StringRef _Sysroot = "")
     : Sysroot(_Sysroot), UseStandardIncludes(true) {}
 
   /// AddPath - Add the \arg Path path to the specified \arg Group list.