Reset PAL metadata when AMDGPU traget stream finishes

If the same stream object is used for multiple compiles, the PAL metadata from eariler compilations will leak into later one.  See https://github.com/GPUOpen-Drivers/llpc/issues/882 for how this is happening in LLPC.

No tests were added because multiple compiles will have to happen using the same pass manager, and I do not see a setup for that on the LLVM side.  Let me know if there is a good way to test this.

Reviewed By: nhaehnle

Differential Revision: https://reviews.llvm.org/D85667