Make the construction of the code-completion string for a function

Make the construction of the code-completion string for a function
template smarter, by taking into account which function template
parameters are deducible from the call arguments. For example,

  template<typename RandomAccessIterator>
  void sort(RandomAccessIterator first, RandomAccessIterator last);

will have a code-completion string like

  sort({RandomAccessIterator first}, {RandomAccessIterator last})

since the template argument for its template parameter is
deducible. On the other hand,

  template<class X, class Y>
  X* dyn_cast(Y *Val);

will have a code-completion string like

  dyn_cast<{class X}>({Y *Val})

since the template type parameter X is not deducible from the function
call.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@82306 91177308-0d34-0410-b5e6-96231b3b80d8