[clang-tools-extra] [clang-tidy] warn when `true` is used as a preprocessor keyword in C (PR #128265)

[Richard Smith via cfe-commits]

zygoloid wrote:

Perhaps we could split off a warning from `-Wundef` for the specific case where the undefined macro name is `true`, and enable it by default? Doing this in a clang-tidy check seems like the wrong place -- this check is much better suited to being implemented as a compiler warning.

https://github.com/llvm/llvm-project/pull/128265