[mlir] ensureRegionTerminator: take OpBuilder

The SingleBlockImplicitTerminator op trait provides a function
`ensureRegionTerminator` that injects an appropriate terminator into the block
if necessary, which is used during operation constructing and parsing.
Currently, this function directly modifies the IR using low-level APIs on
Operation and Block. If this function is called from a conversion pattern,
these manipulations are not reflected in the ConversionPatternRewriter and thus
cannot be undone or, worse, lead to tricky memory errors and malformed IR.
Change `ensureRegionTerminator` to take an instance of `OpBuilder` instead of
`Builder`, and use it to construct the block and the terminator when required.
Maintain overloads taking an instance of `Builder` and creating a simple
`OpBuilder` to use in parsers, which don't have an `OpBuilder` and cannot
interact with the dialect conversion mechanism. This change was one of the
reasons to make `<OpTy>::build` accept an `OpBuilder`.

Differential Revision: https://reviews.llvm.org/D80138