[llvm] [NFC][LLVM] Code cleanup in RegionPrinter (PR #166622)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-nvptx64-nvidia-ubuntu` running on `as-builder-7` while building `llvm` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/160/builds/27856

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (build-unified-tree) failure: build (failure)
...
                 from /home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/include/llvm/CodeGen/ScheduleDAG.h:24,
                 from /home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/include/llvm/CodeGen/MachineScheduler.h:87,
                 from /home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/lib/Passes/PassBuilder.cpp:134:
/home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/include/llvm/CodeGen/LibcallLoweringInfo.h:15:46: warning: ‘visibility’ attribute ignored [-Wattributes]
   15 |   LLVM_ABI const RTLIB::RuntimeLibcallsInfo &RTLCI;
      |                                              ^~~~~
/home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/include/llvm/CodeGen/LibcallLoweringInfo.h:18:25: warning: ‘visibility’ attribute ignored [-Wattributes]
   18 |       RTLIB::Unsupported};
      |                         ^
41.226 [709/8/2136] Building CXX object lib/Analysis/CMakeFiles/LLVMAnalysis.dir/RegionPrinter.cpp.o
FAILED: lib/Analysis/CMakeFiles/LLVMAnalysis.dir/RegionPrinter.cpp.o 
CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros /usr/bin/ccache /usr/bin/c++ -DGTEST_HAS_RTTI=0 -DHAS_LOGF128 -DLLVM_EXPORTS -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_USE_CXX11_ABI=1 -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/build/lib/Analysis -I/home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/lib/Analysis -I/home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/build/include -I/home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-dangling-reference -Wno-redundant-move -Wno-pessimizing-move -Wno-array-bounds -Wno-stringop-overread -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wno-misleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17 -fPIC  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -MD -MT lib/Analysis/CMakeFiles/LLVMAnalysis.dir/RegionPrinter.cpp.o -MF lib/Analysis/CMakeFiles/LLVMAnalysis.dir/RegionPrinter.cpp.o.d -o lib/Analysis/CMakeFiles/LLVMAnalysis.dir/RegionPrinter.cpp.o -c /home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/lib/Analysis/RegionPrinter.cpp
/home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/lib/Analysis/RegionPrinter.cpp:48:8: error: explicit specialization of ‘template<class Ty> struct llvm::DOTGraphTraits’ outside its namespace must use a nested-name-specifier [-fpermissive]
   48 | struct DOTGraphTraits<RegionInfo *>
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
46.417 [709/4/2140] Building CXX object lib/MC/MCParser/CMakeFiles/LLVMMCParser.dir/AsmParser.cpp.o
53.627 [709/2/2142] Building CXX object lib/CodeGen/AsmPrinter/CMakeFiles/LLVMAsmPrinter.dir/AsmPrinter.cpp.o
In file included from /home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/include/llvm/CodeGen/TargetLowering.h:32,
                 from /home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp:62:
/home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/include/llvm/CodeGen/LibcallLoweringInfo.h:15:46: warning: ‘visibility’ attribute ignored [-Wattributes]
   15 |   LLVM_ABI const RTLIB::RuntimeLibcallsInfo &RTLCI;
      |                                              ^~~~~
/home/buildbot/worker/as-builder-7/llvm-nvptx64-nvidia-ubuntu/llvm-project/llvm/include/llvm/CodeGen/LibcallLoweringInfo.h:18:25: warning: ‘visibility’ attribute ignored [-Wattributes]
   18 |       RTLIB::Unsupported};
      |                         ^
58.453 [709/1/2143] Building CXX object lib/LTO/CMakeFiles/LLVMLTO.dir/LTO.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/166622