[flang] add RTBuilder.h

This is a set of type building models that is specific to the lowering
process. It provides the mechanism of mapping C(++) header file interfaces
to the MLIR+FIR type system.

It also provides some macros to build a constexpr evaluated table to
runtime functions. This code is used to build the interface tables to
various runtime support libraries.

Differential revision: https://reviews.llvm.org/D82387