[Support] Add StringRef::withNullAsEmpty()

When porting large bodies of code from using const char*
to StringRef, it is helpful to be able to treat nullptr
as an empty string, since that it is often what it is used
to indicate in C-style code.

Differential Revision: https://reviews.llvm.org/D24697

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281906 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Zachary Turner

include/llvm/ADT/StringRef.h

@@ -88,6 +88,10 @@ namespace llvm {
     /*implicit*/ StringRef(const std::string &Str)
       : Data(Str.data()), Length(Str.length()) {}
 
+    static StringRef withNullAsEmpty(const char *data) {
+      return StringRef(data ? data : "");
+    }
+
     /// @}
     /// @name Iterators
     /// @{