[NFC] Move PaddingClearer functionality into ASTContext (#215823)
CIR needs to use this functionality to support
`__builtin_clear_padding`, and a
`bit-cast-padding` warning could also apparently use it as well. This
patch moves
this logic to a function in `ASTContext`, which returns a vector of
effectively 'pair'
objects. This result is now ALSO cached, so we don't have to generate it
multiple
times for the same objects, as suggested by Eli.
The logic is effectively copy/pasted with some minor changes to remove
the dependency
on CodeGen/LLVM-IR.
The conversion of the 'pair' objects to IR is done in CodeGen, but that
function is generally
only mildly changed.
llvm-project
173f268d - [NFC] Move PaddingClearer functionality into ASTContext (#215823)
