Allow multiple -liberty args in dfflibmap and clockgate

[widlarizer]

dfflibmap and clockgate are the only passes which accept a -liberty flag but only once. This requires workarounds for PDKs like asap7 which come in multiple files sharing the same "header" (that is, anything but cells) and having distinct sets of cells. The other commands accepting any number of -liberty arguments are the abc commands and stat. read_liberty is a frontend and therefore fixed to one file per invocation.

In this PR, I'm extending libparse with LibertyMergedCells which steals LibertyAst* out of LibertyParsers before the AST is destroyed when ~LibertyAst is executed. I'm extending tests for both affected commands to cover this functionality. There's a nice extra benefit of deduplicating some sanity checks from passes into LibertyMergedCells.

[KrystalDelusion]

read_liberty is a frontend and therefore fixed to one file per invocation.

Is that true for all frontends (other than verific which is a special case)? I'm sure I've used read_verilog with multiple files, but maybe that results in multiple calls?

[povik]

I'm sure I've used read_verilog with multiple files, but maybe that results in multiple calls?

I've used read_liberty with multiple files, I think there's external logic to repeatedly call the frontend