[flang] Fold SCALE()

Fold references to the intrinsic function SCALE().

(Also work around some MSVC headaches somehow exposed by
this patch: disable a bogus MSVC warning that began to appear
in unrelated source files, and avoid the otherwise-necessary
use of the "template" keyword in a call to a template member
function of a class template.)

Differential Revision: https://reviews.llvm.org/D117150