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

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.

Differential Revision: https://reviews.llvm.org/D125180

clang/lib/Frontend/PrecompiledPreamble.cpp

@@ -765,6 +765,10 @@ void PrecompiledPreamble::configurePreamble(
   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);
 }