PDB is getting generated in FSI scenarios even if it's not needed

[vzarytovskii]

Also a note - we spend quite some time generating PDB in all scenarios, even though it will never be used. Around 1/3rd of total assembly generation time, for example on my M2 it takes 50ms to generate metadata and additional 24ms to generate pdb.

Originally posted by @vzarytovskii in #17151 (comment)

CC @KevinRansom