[PATCH] Add more information when displaying a "read-only variable is not assignable" error

[Richard Trieu]

http://llvm.org/bugs/show_bug.cgi?id=19139

When a "read-only variable is not assignable" is emitted, give additional notes to show where the const is location.

$ cat test.cc
const int k = 1;
void f() { k = 0; }
class X {
  int i;
  const int j = 1;
  void f() const { i = 0; }
  void g() { j = 0; }
};
$ old_clang test.cc -std=c++11
test.cc:2:14: error: read-only variable is not assignable
void f() { k = 0; }
           ~ ^
test.cc:6:22: error: read-only variable is not assignable
  void f() const { i = 0; }
                   ~ ^
test.cc:7:16: error: read-only variable is not assignable
  void g() { j = 0; }
             ~ ^
3 errors generated.
$ new_clang test.cc -std=c++11
test.cc:2:14: error: read-only variable is not assignable
void f() { k = 0; }
           ~ ^
test.cc:1:11: note: variable 'k' is declared here with type 'const int' which is const
const int k = 1;
~~~~~~~~~~^~~~~
test.cc:6:22: error: read-only variable is not assignable
  void f() const { i = 0; }
                   ~ ^
test.cc:6:8: note: method 'f' is declared const here
  void f() const { i = 0; }
  ~~~~~^~~~~~~~~
test.cc:7:16: error: read-only variable is not assignable
  void g() { j = 0; }
             ~ ^
test.cc:5:13: note: field 'j' is declared here with type 'const int' which is const
  const int j = 1;
  ~~~~~~~~~~^~~~~
3 errors generated.

http://reviews.llvm.org/D4479

Files:
  include/clang/Basic/DiagnosticSemaKinds.td
  lib/Sema/SemaExpr.cpp
  test/CXX/temp/temp.decls/temp.mem/p5.cpp
  test/Sema/block-misc.c
  test/SemaCXX/anonymous-union.cpp
  test/SemaCXX/captured-statements.cpp
  test/SemaCXX/cxx0x-constexpr-const.cpp
  test/SemaCXX/err_typecheck_assign_const.cpp
  test/SemaCXX/function-type-qual.cpp
  test/SemaObjC/arc.m
  test/SemaTemplate/dependent-type-identity.cpp
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