[mlir][EDSC] Make use of InsertGuard

Summary:
This revision cleans up a layer of complexity in ScopedContext and uses InsertGuard instead of previously manual bookkeeping.
The method `getBuilder` is renamed to `getBuilderRef` and spurious copies of OpBuilder are tracked.

This results in some canonicalizations not happening anymore in the Linalg matmul to vector test. This test is retired because relying on DRRs for this has been shaky at best. The solution will be better support to write fused passes in C++ with more idiomatic pattern composition and application.

Differential Revision: https://reviews.llvm.org/D79208