Add `replace` interface with range in AtomicChange.

[ioeric]

Please see comment from a previous patch (https://reviews.llvm.org/D27054?id=79099#inline-234270). Generally, AtomicChange is a higher level of abstraction than Replacement, and we don't want users to deal with Replacement and the related errors.

[hokein]

Get rid of Replacement, and implement "replace" interface with range.

[alexander-shaposhnikov]

to avoid misunderstanding - are the tools like clang-rename, change-namespace and clang-reorder-fields supposed to use AtomicChange (via the methods insert, replace etc) ?
(i am asking just to make sure i understand correctly the intentions behind AtomicChange interface)

[ioeric]

In D31492#713908, @alexshap wrote:

to avoid misunderstanding - are the tools like clang-rename, change-namespace and clang-reorder-fields supposed to use AtomicChange (via the methods insert, replace etc) ?
(i am asking just to make sure i understand correctly the intentions behind AtomicChange interface)

In the future, yes. We have a plan to redesign ClangTool interface and the refactoring infrastructure (e.g. to support multi-TU/codebase-wide refactoring) which would use AtomicChange interface in the user level.

[hokein]

Rebase to master.

[ioeric]

Could you add a simple test?

[hokein]

Add unittest.

[ioeric]

Looks good and thanks for the cleanup.

[hokein]

are the tools like clang-rename, change-namespace and clang-reorder-fields supposed to use AtomicChange (via the methods insert, replace etc) ?

Yeah. We are using AtomicChange in clang-rename refinement (D31176).

include/clang/Tooling/Refactoring/AtomicChange.h

public:
   void setError(llvm::StringRef Error) { this->Error = Error; }
 
   /// \brief Returns whether an error has been set on this list.
   bool hasError() const { return !Error.empty(); }
 
   /// \brief Returns the error message or an empty string if it does not exist.
   const std::string &getError() const { return Error; }
 
+  /// \brief Adds a replacement to the atomic change.
+  llvm::Error addReplacement(const Replacement& R);
+
   /// \brief Adds a replacement that replaces range [Loc, Loc+Length) with
   /// \p Text.
   /// \returns An llvm::Error carrying ReplacementError on error.
   llvm::Error replace(const SourceManager &SM, SourceLocation Loc,
                       unsigned Length, llvm::StringRef Text);
 
   /// \brief Adds a replacement that inserts \p Text at \p Loc. If this
   /// insertion conflicts with an existing insertion (at the same position),

lib/Tooling/Refactoring/AtomicChange.cpp

for (const auto &R : NE.Replaces) {
     if (Err)
       llvm_unreachable(
           "Failed to add replacement when Converting YAML to AtomicChange.");
     llvm::consumeError(std::move(Err));
   }
   return E;
 }
 
+llvm::Error AtomicChange::addReplacement(const Replacement& R) {
+  return Replaces.add(R);
+}
+
 llvm::Error AtomicChange::insert(const SourceManager &SM, SourceLocation Loc,
                                  llvm::StringRef Text, bool InsertAfter) {
   if (Text.empty())
     return llvm::Error::success();
   Replacement R(SM, Loc, 0, Text);
   llvm::Error Err = Replaces.add(R);
   if (Err) {
     return llvm::handleErrors(