[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