Explicitly delete fbstring_core copy-assignment (#1123)

Summary:
- Replace `fbstring_core` private copy assignment special member function
  with an explicit `=delete` equivalent.
Pull Request resolved: https://github.com/facebook/folly/pull/1123

Reviewed By: Orvid

Differential Revision: D15216597

Pulled By: yfeldblum

fbshipit-source-id: 319109f739d972e9efc73a7dbc2dd6ae401b18fc

folly/FBString.h

@@ -197,6 +197,7 @@ class fbstring_core_model {
  public:
   fbstring_core_model();
   fbstring_core_model(const fbstring_core_model &);
+  fbstring_core_model& operator=(const fbstring_core_model &) = delete;
   ~fbstring_core_model();
   // Returns a pointer to string's buffer (currently only contiguous
   // strings are supported). The pointer is guaranteed to be valid
@@ -243,9 +244,6 @@ class fbstring_core_model {
   // the string without reallocation. For reference-counted strings,
   // it should fork the data even if minCapacity < size().
   void reserve(size_t minCapacity);
- private:
-  // Do not implement
-  fbstring_core_model& operator=(const fbstring_core_model &);
 };
 */
 
@@ -311,6 +309,8 @@ class fbstring_core {
     FBSTRING_ASSERT(memcmp(data(), rhs.data(), size() * sizeof(Char)) == 0);
   }
 
+  fbstring_core& operator=(const fbstring_core& rhs) = delete;
+
   fbstring_core(fbstring_core&& goner) noexcept {
     // Take goner's guts
     ml_ = goner.ml_;
@@ -483,9 +483,6 @@ class fbstring_core {
   }
 
  private:
-  // Disabled
-  fbstring_core& operator=(const fbstring_core& rhs);
-
   Char* c_str() {
     Char* ptr = ml_.data_;
     // With this syntax, GCC and Clang generate a CMOV instead of a branch.