[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 `bolt-x86_64-ubuntu-nfc` running on `bolt-worker` while building `llvm` at step 7 "build-bolt".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/92/builds/26977

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

```
Step 7 (build-bolt) failure: build (failure)
0.010 [14/3/1] Performing build step for 'bolt_rt'
ninja: no work to do.
0.013 [13/3/2] Generating VCSRevision.h
0.016 [7/4/3] Generating VCSVersion.inc
0.021 [5/3/4] No install step for 'bolt_rt'
0.036 [4/3/5] Completed 'bolt_rt'
1.907 [4/2/6] Building CXX object lib/Analysis/CMakeFiles/LLVMAnalysis.dir/RegionPrinter.cpp.o
FAILED: lib/Analysis/CMakeFiles/LLVMAnalysis.dir/RegionPrinter.cpp.o 
ccache /usr/bin/c++ -DGTEST_HAS_RTTI=0 -DHAS_LOGF128 -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/worker/bolt-worker2/bolt-x86_64-ubuntu-nfc/build/lib/Analysis -I/home/worker/bolt-worker2/bolt-x86_64-ubuntu-nfc/llvm-project/llvm/lib/Analysis -I/home/worker/bolt-worker2/bolt-x86_64-ubuntu-nfc/build/include -I/home/worker/bolt-worker2/bolt-x86_64-ubuntu-nfc/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-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  -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -std=c++17 -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/worker/bolt-worker2/bolt-x86_64-ubuntu-nfc/llvm-project/llvm/lib/Analysis/RegionPrinter.cpp
/home/worker/bolt-worker2/bolt-x86_64-ubuntu-nfc/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 *>
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
3.302 [4/1/7] Building CXX object lib/Analysis/CMakeFiles/LLVMAnalysis.dir/RegionInfo.cpp.o
ninja: build stopped: subcommand failed.

```

</details>

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