Fix modules build with different technique to suppress Knuth debugging

Currently we use #pragma push_macro(LLVM_DEBUG) to fiddle with the LLVM_DEBUG
macro so that we can silence debugging the Knuth division algorithm unless it's
actually desired. Unfortunately this is incompatible with enabling modules
while building LLVM (via LLVM_ENABLE_MODULES=ON), probably due to a bug being
fixed by D33004.

llvm-svn: 339009