[flang][NFC] Remove CallBuilder class

The methods of CallBuilder do need to belong to a class.
This was made to avoid having to propagate generic lowering context
(converter, symbol mappings, location and StatementContext).

Packaging them together will actually make it harder to share the code
for user and intrinsic elemental lowering (I plan to use C++ CRTP),
and it is also misleading: one could think there is something going
with the class state while lowering the function while there is not
(and there should not be).

Removes the class and turns the methods into static functions.
Add a new CallContext class to solve the argument threading
inconvenience.

This contains no functional changes at all.

Differential Revision: https://reviews.llvm.org/D141510