build-script: Don't automatically clear the module cache.

On Darwin, LLVM builds with -gmodules which causes debug info for the
types defined by the LLVM modules is stored alongside the modules in the
module cache. Deleting the module cache on every re-invocation of
build-script without re-building the LLVM and clang modules will erase
the debug info and thus render any LLVM types undebuggable.

rdar://problem/28655564

Author: adrian-prantl

utils/build-script-impl

@@ -2442,10 +2442,6 @@ for host in "${ALL_HOSTS[@]}"; do
                 ;;
         esac
 
-        # Clean the product-local module cache.
-        call rm -rf "${module_cache}"
-        call mkdir -p "${module_cache}"
-
         # Compute the generator output file to check for, to determine if we
         # must reconfigure. We only handle Ninja for now.
         #