[mlir] Provide OpBuilder-based replacements for edsc::BlockBuilder

The ScopedBuilder class in EDSC is being gradually phased out in favor of core
OpBuilder-based helpers with callbacks. Provide helper functions that are
compatible with `edsc::ScopedContext` and can be used to create and populate
blocks using callbacks that take block arguments as callback arguments. This
removes the need for `edsc::BlockHandle`, forward-declaration of `Value`s used
for block arguments and the tag `edsc::Append` class, leading to noticable
reduction in the verbosity of the code using helper functions.

Remove "eager mode" construction tests that are only relevant to the
`BlockBuilder`-based approach.

`edsc::BlockHandle` and `edsc::BlockBuilder` are now deprecated and will be
removed soon.

Differential Revision: https://reviews.llvm.org/D82008