[flang] Add the complex expression helper class.

The complex expression helper class can be used to generate small,
short-lived instances of a "helper" that can be used to aid the
construction of complex expressions in FIR. The helper class bundles
together these functionally related operations.

Included in this diff is the header for the FIR builder. The
implementation has other dependences and will follow.

Differential Revision: https://reviews.llvm.org/D81287