[ms] Add new /PDBSTREAM option to lld-link allowing injection of streams into PDB files.

Summary:
/PDBSTREAM:<name>=<file> adds the contents of <file> to stream <name> in the resulting PDB.

This allows native uses with workflows that (for example) add srcsrv streams to PDB files to provide a location for the build's source files.

Results should be equivalent to linking with lld-link, then running Microsoft's pdbstr tool with the command line:
pdbstr.exe -w -p:<PDB LOCATION> -s:<name> -i:<file>
except in cases where the named stream overlaps with a default named stream, such as "/names". In those cases, the added stream will be overridden, making the /pdbstream option a no-op.

Reviewers: thakis, rnk

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D77310