[SmallString] Use data() instead of begin() (NFC)

Both begin() and data() do the same thing for the SmallString case, but
the std::string and llvm::StringRef constructors that are being called
are defined as taking a pointer and size.

Addresses Craig Topper's feedback in https://reviews.llvm.org/D73640

llvm/include/llvm/ADT/SmallString.h

@@ -263,7 +263,7 @@ public:
   // Extra methods.
 
   /// Explicit conversion to StringRef.
-  StringRef str() const { return StringRef(this->begin(), this->size()); }
+  StringRef str() const { return StringRef(this->data(), this->size()); }
 
   // TODO: Make this const, if it's safe...
   const char* c_str() {
@@ -276,7 +276,7 @@ public:
   operator StringRef() const { return str(); }
 
   explicit operator std::string() const {
-    return std::string(this->begin(), this->size());
+    return std::string(this->data(), this->size());
   }
 
   // Extra operators.