Revert "[Clang] Move assembler into a separate file"

This reverts commit 2325157c.

Unfortunately this commit produces linker errors on some builds:
http://lab.llvm.org:8011/#/builders/57/builds/3704
http://lab.llvm.org:8011/#/builders/112/builds/3216
http://lab.llvm.org:8011/#/builders/121/builds/3900