[llvm] Add __attribute__((retain)) to LLVM_DUMP_METHOD (PR #133025)

[Aleksandr Popov via llvm-commits]

aleks-tmb wrote:

Hi @MatzeB, this change triggered multiple `‘retain’ attribute ignored` warnings when building LLVM with gcc-13:
```
$ gcc --version
gcc (Ubuntu 13.1.0-8ubuntu1~20.04.2) 13.1.0
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
```
```
...
In file included from /home/apopov/llvm-project/llvm/utils/TableGen/Common/GlobalISel/GlobalISelMatchTable.h:26,
                 from /home/apopov/llvm-project/llvm/utils/TableGen/Common/GlobalISel/GlobalISelMatchTable.cpp:9:
/home/apopov/llvm-project/llvm/include/llvm/CodeGenTypes/LowLevelType.h:288:32: warning: ‘retain’ attribute ignored [-Wattributes]
  288 |   LLVM_DUMP_METHOD void dump() const;
      |                                ^~~~~
In file included from /home/apopov/llvm-project/llvm/utils/TableGen/Common/GlobalISel/GlobalISelMatchTable.h:26,
                 from /home/apopov/llvm-project/llvm/utils/TableGen/Common/GlobalISel/GlobalISelMatchTableExecutorEmitter.cpp:10:
/home/apopov/llvm-project/llvm/include/llvm/CodeGenTypes/LowLevelType.h:288:32: warning: ‘retain’ attribute ignored [-Wattributes]
  288 |   LLVM_DUMP_METHOD void dump() const;
```

https://github.com/llvm/llvm-project/pull/133025