[AMDGPU][OpenCL] Remove "printf and hostcall" diagnostic

The diagnostic is unreliable, and triggers even for dead uses of
hostcall that may exist when linking the device-libs at lower
optimization levels.

Eliminate the diagnostic, and directly document the limitation for
OpenCL before code object V5.

Make some NFC changes to clarify the related code in the
MetadataStreamer.

Add a clang test to tie OCL sources containing printf to the backend IR
tests for this situation.

Reviewed By: sameerds, arsenm, yaxunl

Differential Revision: https://reviews.llvm.org/D121951