[NFC] Convert ParsedAttr to use llvm::TrailingObjects

ParsedAttr is using a hand-rolled trailing-objects
implementation that gets cleaned up quite a bit by
just using llvm::TrailingObjects. This is a large
TrailingObjects list, but most things are length '0'.

Differential Revision: https://reviews.llvm.org/D50531

llvm-svn: 339380