[EDSC] Cleanup declarative builder insertion point with blocks

[EDSC] Cleanup declarative builder insertion point with blocks

Declarative builders want to provide the same nesting interface for blocks and loops. MLIR on the other hand has different behaviors:
1. when an AffineForOp is created the insertion point does not enter the loop body;
2. when a Block is created, the insertion point does enter the block body.

Guard against the second behavior in EDSC to make the interface unsurprising.
This also surfaces two places in the eager branch API where I was guarding against this behavior indirectly by creating a new ScopedContext.

Instead, uniformize everything to properly reset the insertion point in the unique place that builds the mlir::Block*.

PiperOrigin-RevId: 237619513