[clangd] Disable predefined macros in tests. NFC

[sammccall]

Summary

These aren't needed. With them the generated predefines buffer is 13KB.
For every TestTU, we must:

• generate the buffer (3 times: parsing preamble, scanning preamble, main file)
• parse the buffer (again 3 times)
• serialize all the macros it defines in the PCH
• compress the buffer itself to write it into the PCH
• decompress it from the PCH

Avoiding this reduces unit test time by ~25%.

[ilya-biryukov]

LGTM.

clang-tools-extra/clangd/unittests/TestTU.cpp

44

NIT: maybe mention that not doing this actually saves a big fraction of test time. It might not be obvious that this type of work is that expensive even for hundreds of predefined macros.

clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp

 
 TEST(IncludeCleaner, VirtualBuffers) {
   TestTU TU;
   TU.Code = R"cpp(
     #include "macros.h"
 
     using flags::FLAGS_FOO;
 
-    // CLI will come from a define, __llvm__ is a built-in. In both cases, they
+    // CLI will come from a define, __cplusplus is a built-in. In both cases, they
     // come from non-existent files.
-    int y = CLI + __llvm__;
+    int y = CLI + __cplusplus;
 
     int concat(a, b) = 42;
     )cpp";
   // The pasting operator in combination with DEFINE_FLAG will create
   // ScratchBuffer with `flags::FLAGS_FOO` that will have FileID but not
   // FileEntry.
   TU.AdditionalFiles["macros.h"] = R"cpp(
     #ifndef MACROS_H

clang-tools-extra/clangd/unittests/TestTU.cpp

ParseInputs TestTU::inputs(MockFS &FS) const {
   FS.Files = AdditionalFiles;
   FS.Files[FullFilename] = Code;
   FS.Files[FullHeaderName] = HeaderCode;
   FS.Files[ImportThunk] = ThunkContents;
 
   ParseInputs Inputs;
   Inputs.FeatureModules = FeatureModules;
   auto &Argv = Inputs.CompileCommand.CommandLine;
-  Argv = {"clang"};
+  // In tests, we don't need predefined macros (__GNUC__, __CHAR_BIT__) etc.
+  // There are hundreds, and we'd generate, parse, serialize, and re-parse them!
+  Argv = {"clang", "-Xclang", "-undef"};
   // FIXME: this shouldn't need to be conditional, but it breaks a
   // GoToDefinition test for some reason (getMacroArgExpandedLocation fails).
   if (!HeaderCode.empty()) {
     Argv.push_back("-include");
     Argv.push_back(ImplicitHeaderGuard ? ImportThunk : FullHeaderName);
     // ms-compatibility changes the meaning of #import.
     // The default is OS-dependent (on on windows), ensure it's off.
     if (ImplicitHeaderGuard)