EPA: Deal with header comments attached to first module decl
After parsing, fix up the header comments by looking for a where
keyword and moving any comments in the first declaration that precede
it into the header comments.
This only affects a specific scenario where
- There are comments in the lines before the module header
- There is a comment on the same line as the 'where', preceding it
- There is a decl immediately following the line with the comment and 'where' token
Closes #23984
Note: there is no need to backport to GHC 9.8.1, as the fix is already in ghc-exactprint.