Mention Devirtualization and ThinLTO in ReleaseNotes

  clang to emit a warning or error if a function call meets one or more

  user-specified conditions.

- Enhanced devirtualization with

  '-fstrict-vtable-pointers <http://clang.llvm.org/docs/UsersManual.html#cmdoption-fstrict-vtable-pointers>`_.

  Clang devirtualizes across different basic blocks, like loops:

  .. code-block:: c++

       struct A {

           virtual void foo();

       };

       void indirect(A &a, int n) {

           for (int i = 0 ; i < n; i++)

               a.foo();

       }

       void test(int n) {

           A a;

           indirect(a, n);

       }

-  ...

Improvements to ThinLTO (-flto=thin)

------------------------------------

- Integration with profile data (PGO). When available, profile data enables

  more accurate function importing decisions, as well as cross-module indirect

  call promotion.

- Significant build-time and binary-size improvements when compiling with debug

  info (-g).

Improvements to Clang's diagnostics

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