Fix FBStringTest build failure and hash computation for wchar

Summary:
Getting some build errors when doing `buck test folly/...`. This seems to be fixing it.

  folly/test/FBStringTest.cpp:1290:23: error: unknown type name 'basic_fbstring'; did you mean 'basic_fstream'?
    using u16fbstring = basic_fbstring<char16_t>;
                        ^~~~~~~~~~~~~~
                        basic_fstream

Also the test `testHashChar16` was failing because only part of the string is used when computing hash.

Reviewed By: yfeldblum

Differential Revision: D3575858

fbshipit-source-id: cdd5bdb9653d50beaf0ec82b659d31354b345441

folly/FBString.h

@@ -2438,12 +2438,12 @@ _GLIBCXX_END_NAMESPACE_VERSION
 // Handle interaction with different C++ standard libraries, which
 // expect these types to be in different namespaces.
 
-#define FOLLY_FBSTRING_HASH1(T) \
-  template <> \
-  struct hash< ::folly::basic_fbstring<T> > { \
-    size_t operator()(const ::folly::basic_fbstring<T>& s) const { \
-      return ::folly::hash::fnv32_buf(s.data(), s.size()); \
-    } \
+#define FOLLY_FBSTRING_HASH1(T)                                        \
+  template <>                                                          \
+  struct hash<::folly::basic_fbstring<T>> {                            \
+    size_t operator()(const ::folly::basic_fbstring<T>& s) const {     \
+      return ::folly::hash::fnv32_buf(s.data(), s.size() * sizeof(T)); \
+    }                                                                  \
   };
 
 // The C++11 standard says that these four are defined

folly/test/FBStringTest.cpp

@@ -1287,7 +1287,7 @@ TEST(FBString, testHash) {
 
 #if FOLLY_HAVE_WCHAR_SUPPORT
 TEST(FBString, testHashChar16) {
-  using u16fbstring = basic_fbstring<char16_t>;
+  using u16fbstring = folly::basic_fbstring<char16_t>;
   u16fbstring a;
   u16fbstring b;
   a.push_back(0);