[clang-tidy] Enable 'bugprone-unused-return-value' check in clang-tidy config.

[vbvictor]

The check could be enabled in .clang-tidy or given reason why it shouldn't

All the violations in clang-tools-extra/clang-tidy directory should be fixed.

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Baranov Victor (vbvictor)

The check could be enabled in [.clang-tidy](https://github.com/llvm/llvm-project/blob/main/clang-tools-extra/clang-tidy/.clang-tidy) or given reason why it shouldn't

[vbvictor]

Currently, all violations of this check involve operations with filesystem:

/home/victor/llvm/llvm-project/clang-tools-extra/clang-tidy/ClangTidy.cpp:197:9: warning: the value returned by this function should not be disregarded; neglecting it may lead to errors [bugprone-unused-return-value]
  197 |         VFS.setCurrentWorkingDirectory(FileAndReplacements.second.BuildDir);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/victor/llvm/llvm-project/clang-tools-extra/clang-tidy/ClangTidy.cpp:243:9: warning: the value returned by this function should not be disregarded; neglecting it may lead to errors [bugprone-unused-return-value]
  243 |         VFS.setCurrentWorkingDirectory(*OriginalCWD);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/victor/llvm/llvm-project/clang-tools-extra/clang-tidy/ClangTidy.cpp:642:7: warning: the value returned by this function should not be disregarded; neglecting it may lead to errors [bugprone-unused-return-value]
  642 |       FileSystem.setCurrentWorkingDirectory(Error.BuildDirectory);
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/victor/llvm/llvm-project/clang-tools-extra/clang-tidy/ClangTidy.cpp:646:5: warning: the value returned by this function should not be disregarded; neglecting it may lead to errors [bugprone-unused-return-value]
  646 |     FileSystem.setCurrentWorkingDirectory(InitialWorkingDir.get());
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In most of the places, there is no good way to recover from these errors, so I'd keep the check disabled for now.