R600: Fix up for AsmPrinter's OutStreamer being a unique_ptr

Fixes a crash with basically any OpenGL application using the radeonsi
driver.

Patch by: Michel Dänzer

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90176


Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
llvm-svn: 236004