[flang] Support lowering of intrinsic module procedure C_F_POINTER

As Fortran 2018 18.2.3.3, the intrinsic module procedure
C_F_POINTER(CPTR, FPTR [, SHAPE]) associates a data pointer with the
target of a C pointer and specify its shape. CPTR shall be a scalar of
type C_PTR, and its value is the C address or the result of a reference
to C_LOC. FPTR is one pointer, either scalar or array. SHAPE is a
rank-one integer array, and it shall be present if and only if FPTR is
an array.

C_PTR is the derived type with only one component of integer 64, and the
integer 64 component value is the address. Build the right "source"
fir::ExtendedValue based on the address and shape, and use
associateMutableBox to associate the pointer with the target of the C
pointer.

Refactor the getting the address of C_PTR to reuse the code.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D132303