[clang] Add ParserConversionAction (PR #169803)

[Abhina Sree via cfe-commits]

abhina-sree wrote:

> @abhina-sree I left a bunch of comments here related to format string handling, but that functionality was later extracted into #196568 and the comments have become outdated, which is unfortunate. I'm not sure if they can be unoutdated by pulling the changes back, so please just to make sure to skim through them.

Thanks for reminding me, I will make sure to finish going through them for the format string handling PR

https://github.com/llvm/llvm-project/pull/169803