[clang] [clang-format] Limit how much work guessLanguage() can do (PR #78925)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> The OOM is not limited to `guessLanguage()`. [...]  It occurs even if you rename the header files to .cpp files and clang-format the renamed files directly.

I'm aware of that.

However, for clangd users, a crucial difference between `guessLanguage()` vs. other code in libFormat that uses `UnwrappedLineParser`, is that `guessLanguage()` gets called as soon as a file is opened in the editor, while the other code is only called if the user explicitly tries to format the file.

For the files affected by this OOM (single-file libraries like [miniaudio](https://github.com/mackron/miniaudio/blob/master/miniaudio.h), [mathlink](https://github.com/clangd/clangd/issues/1384), [minilzo](http://www.oberhumer.com/opensource/lzo/#minilzo), [blis](https://github.com/llvm/llvm-project/issues/60151), where the large number of configuration-related preprocessor branches and the large number of lines in the file conspire to make the combined length of all permutations intractable to work with), the number of users who merely use these libraries (and so may open them in the editor to look at the header, but will not try to format it) far exceeds the number of users who actually develop these libraries (who would edit and format the header). Thus, by avoiding the problem in `guessLanguage()`, we would resolve the issue for >99% of clangd users that run into it, which would be a significant improvement.

> (See [119a728](https://github.com/llvm/llvm-project/commit/119a72866f0e143127355fe6e03f57c4f8bab1ae) for an improvement to the performance of `guessLanguage()`.)

Thanks for this improvement. However, it does not meaningfully help avoid the OOM for the above libaries (the OOM happens while we're still in [`UnwrappedLineParser::parse()`](https://github.com/llvm/llvm-project/blob/04fbc461e0fd1c6f2b014761e9c03ca80d17b33b/clang/lib/Format/TokenAnalyzer.cpp#L113)).

https://github.com/llvm/llvm-project/pull/78925