[clang-tidy] misc-unconventional-assign-operator suggest to use rvalue references in C++03 mode

Summary:
Bugzilla: https://bugs.llvm.org/show_bug.cgi?id=27702
I wasn't sure how this type of thing is usually tested. So any advice would be appreciated.
`check-llvm`, `check-clang` and `check-clang-tools` are clean for me.
**C++98**
```
tetsuo@garland-c-16-sgp1-01:~/dev/llvm-project/test$ cat compile_commands.json
[
{
  "directory": "/home/tetsuo/dev/llvm-project/test",
  "command": "/usr/bin/c++      -std=gnu++98 -o CMakeFiles/test.dir/test.cpp.o -c /home/tetsuo/dev/llvm-project/test/test.cpp",
  "file": "/home/tetsuo/dev/llvm-project/test/test.cpp"
}
]
tetsuo@garland-c-16-sgp1-01:~/dev/llvm-project/test$ ../build/bin/clang-tidy --checks=misc-unconventional-assign-operator test.cpp
3053 warnings generated.
/home/tetsuo/dev/llvm-project/test/test.cpp:7:3: warning: operator=() should take 'Foo const&' or 'Foo' [misc-unconventional-assign-operator]
  Foo &operator=(Foo &Other) {
  ^
Suppressed 3052 warnings (3052 in non-user code).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.
```
**C++17**
```
tetsuo@garland-c-16-sgp1-01:~/dev/llvm-project/test$ cat compile_commands.json
[
{
  "directory": "/home/tetsuo/dev/llvm-project/test",
  "command": "/usr/bin/c++      -std=gnu++17 -o CMakeFiles/test.dir/test.cpp.o -c /home/tetsuo/dev/llvm-project/test/test.cpp",
  "file": "/home/tetsuo/dev/llvm-project/test/test.cpp"
}
]
tetsuo@garland-c-16-sgp1-01:~/dev/llvm-project/test$ ../build/bin/clang-tidy --checks=misc-unconventional-assign-operator test.cpp
5377 warnings generated.
/home/tetsuo/dev/llvm-project/test/test.cpp:7:3: warning: operator=() should take 'Foo const&', 'Foo&&' or 'Foo' [misc-unconventional-assign-operator]
  Foo &operator=(Foo &Other) {
  ^
Suppressed 5376 warnings (5376 in non-user code).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.
```

Reviewers: njames93, MaskRay, alexfh, hokein, aaron.ballman

Reviewed By: njames93

Subscribers: xazax.hun, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D75901

clang-tools-extra/clang-tidy/misc/UnconventionalAssignOperatorCheck.cpp

@@ -75,7 +75,10 @@ void UnconventionalAssignOperatorCheck::check(
   } else {
     static const char *const Messages[][2] = {
         {"ReturnType", "operator=() should return '%0&'"},
-        {"ArgumentType", "operator=() should take '%0 const&', '%0&&' or '%0'"},
+        {"ArgumentType",
+         getLangOpts().CPlusPlus11
+             ? "operator=() should take '%0 const&', '%0&&' or '%0'"
+             : "operator=() should take '%0 const&' or '%0'"},
         {"cv", "operator=() should not be marked '%1'"}};
 
     const auto *Method = Result.Nodes.getNodeAs<CXXMethodDecl>("method");

clang-tools-extra/test/clang-tidy/checkers/misc-unconventional-assign-operator-precxx11.cpp

@@ -0,0 +1,6 @@
+// RUN: %check_clang_tidy -std=c++98,c++03 %s misc-unconventional-assign-operator %t
+
+struct BadArgument {
+  BadArgument &operator=(BadArgument &);
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: operator=() should take 'BadArgument const&' or 'BadArgument'
+};