[flang] Add Win32 to the list of supported triples

This patch adds Win32 to the list of supported triples in
`fir::CodeGenSpecifics`. This change means that we can use the "native"
triple, even when running tests on Windows. Currently this affects only
1 test, but it will change once we start adding more tests for lowering
and code-generation.

Differential Revision: https://reviews.llvm.org/D119332