[clangd] Skip (most) predefined macros when scanning for preamble patching.

[sammccall]

Summary

This is unneccesary work.
With this change, we skip generating and lexing ~10k of predefines twice.

A dumb benchmark of building a preamble for an empty file in a loop shows:

• before: 1.90ms/run
• after: 1.36ms/run

So this should be worth 0.5ms for each AST build and code completion.

There can be a functional difference, but it's very minor.
If the preamble contains e.g. #ifndef __llvm__ ... #endif then before we would
not take it. After this change we will take the branch (single-file mode takes
all branches with unknown conditions) and so gather different directives.

However I think this is negligible:

• this is already true of non-builtin macros (from included headers). We've had no complaints.
• this affects the baseline and modified in the same way, so only makes a difference transiently when code guarded by such an #ifdef is being edited

[sammccall]

(yes, there's a perverse pleasure in writing 20 lines of commit message for one line of code with no comments. I think in *hindsight* this is obvious enough that it's not worth describing in the code)

[kadircet]

Thx, LGTM. I agree that figuring out includes/macros in conditional branches is already best-effort, so I don't think this will result in noticeable regressions.

clang-tools-extra/clangd/Preamble.cpp

auto Clang = prepareCompilerInstance(
       std::move(CI), nullptr, std::move(PreambleContents),
       // Provide an empty FS to prevent preprocessor from performing IO. This
       // also implies missing resolved paths for includes.
       FS.view(llvm::None), IgnoreDiags);
   if (Clang->getFrontendOpts().Inputs.empty())
     return error("compiler instance had no inputs");
   // We are only interested in main file includes.
   Clang->getPreprocessorOpts().SingleFileParseMode = true;
+  Clang->getPreprocessorOpts().UsePredefines = false;
   PreprocessOnlyAction Action;
   if (!Action.BeginSourceFile(*Clang, Clang->getFrontendOpts().Inputs[0]))
     return error("failed BeginSourceFile");
   Preprocessor &PP = Clang->getPreprocessor();
   IncludeStructure Includes;
   Includes.collect(*Clang);
   ScannedPreamble SP;
   SP.Bounds = Bounds;