[clang-tidy] Fix lint warning in ClangTidyDiagnosticConsumer.cpp (NFC)

Calling clang-tidy on ClangTidyDiagnosticConsumer.cpp gives a
"unmatched NOLINTBEGIN without a subsequent NOLINTEND" warning.

The "NOLINTBEGIN" and "NOLINTEND" string literals used in the
implementation of `createNolintError()` get mistaken for actual
NOLINTBEGIN/END comments used to suppress clang-tidy warnings.

Rewrite the string literals so that they can no longer be mistaken for
actual suppression comments.

Differential Revision: https://reviews.llvm.org/D113472