The previously used syntax, -Wl,-pdb,<file>, was the originally
supported one in lld (since 2018). Later (in 2019) the second
syntax was added, allowing both -Wl,-pdb=<file> and -Wl,-pdb,<file>.
(This other parameter syntax allows easier distinguishing an empty
argument, for letting the linker implicitly pick the file name.)

Move over to using the more modern syntax - reducing the usage of
the old form of the option. This potentially allows deprecating
the original syntax and maybe allows using --pdb as a boolean
flag for implying an automatically named PDB file (which currently
requires using the awkward syntax "--pdb=").

Also prefer the long two dashes form, i.e. --pdb instead of -pdb;
the single dash form is only allowed by getopt when there
are no conflicts with single-letter options, while the form with
two dashes is unambiguous.