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

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

Differential Revision: https://reviews.llvm.org/D125179

clang-tools-extra/clangd/Preamble.cpp

@@ -302,6 +302,7 @@ scanPreamble(llvm::StringRef Contents, const tooling::CompileCommand &Cmd) {
     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");