[Frontend] when attaching a preamble, don't generate the long predefines buffer.

[sammccall]

Summary

We know we're going to overwrite it anyway.
It'd be a bit of work to coordinate not generating it at all, but setting this
flag avoids generating ~10k of the 13k string.

[kadircet]

clang/lib/Frontend/PrecompiledPreamble.cpp

769

s/The/Preamble

to almost nothing

This feels like too much of a detail, i'd probably drop. up to you though.

clang/lib/Frontend/PrecompiledPreamble.cpp

void PrecompiledPreamble::configurePreamble(
 
   // Configure ImpicitPCHInclude.
   PreprocessorOpts.PrecompiledPreambleBytes.first = Bounds.Size;
   PreprocessorOpts.PrecompiledPreambleBytes.second =
       Bounds.PreambleEndsAtStartOfLine;
   PreprocessorOpts.DisablePCHOrModuleValidation =
       DisableValidationForModuleKind::PCH;
 
+  // Don't bother generating the long version of the predefines buffer.
+  // The preamble is going to overwrite it anyway.
+  PreprocessorOpts.UsePredefines = false;
+
   setupPreambleStorage(*Storage, PreprocessorOpts, VFS);
 }
 
 void PrecompiledPreamble::setupPreambleStorage(
     const PCHStorage &Storage, PreprocessorOptions &PreprocessorOpts,
     IntrusiveRefCntPtr<llvm::vfs::FileSystem> &VFS) {
   if (Storage.getKind() == PCHStorage::Kind::TempFile) {
     llvm::StringRef PCHPath = Storage.filePath();