[Windows][Polly] Disable LLVMPolly module for all compilers on Windows

Before this patch, the cmake disabled loadable modules when compiling
with Visual Studio. However, the reason for this is a limitation of the
Windows DLLs, thus this restriction should apply to any compiler for the
Windows platform, such as MinGW, Cygwin, icc, etc.

Differential Revision: https://reviews.llvm.org/D87524