[clangd] Fix crash in bugprone-bad-signal-to-kill-thread clang-tidy check.

[clangd] Fix crash in bugprone-bad-signal-to-kill-thread clang-tidy check.

Inside clangd, clang-tidy checks don't see preprocessor events in the preamble.
This leads to `Token::PtrData == nullptr` for tokens that the macro is defined to.
E.g. `#define SIGTERM 15`:
- Token::Kind == tok::numeric_constant (Token::isLiteral() == true)
- Token::UintData == 2
- Token::PtrData == nullptr

As the result of this, bugprone-bad-signal-to-kill-thread check crashes at null-dereference inside clangd.

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D85417