[mlir] DialectConversion: support erasing blocks

PatternRewriter has support for erasing a Block from its parent region, but
this feature has not been implemented for ConversionPatternRewriter that needs
to keep track of and be able to undo block actions. Introduce support for
undoing block erasure in the ConversionPatternRewriter by marking all the ops
it contains for erasure and by detaching the block from its parent region. The
detached block is stored in the action description and is not actually deleted
until the rewrites are applied.

Differential Revision: https://reviews.llvm.org/D80135