Opinions on naming options wanted!
When integrating PDB output in mingw targeting build systems, it might be a lot of extra work to specify unique file names for the pdb output. Therefore allow omitting the actual file name and let it implicitly be the same name as the linker output, with a pdb extension.
As the current form of the pdb option takes a separate parameter value, e.g. "-pdb out.pdb", it is impractical to leave out the parameter value.
Therefore, introduce a second syntax for the option, with an equals sign, like -pdb=out.pdb, where the value easily can be omitted.
The form -pdb= for requesting pdb files with an implicit name should work fine, even though it looks a bit unconventional in that form.
Alternatively, a flag with a different name could be introduced (e.g. --default-pdb), or we could consider changing the flag name altogether (having -pdb be the default with an implicit name, and "--out-pdb out.pdb" for the case when naming the output file, just like the existing --out-implib). The current number of users of the lld specific pdb option is probably low, so in case renaming the option altogether is deemed the best path forward, that could also be doable.