Only call FlushFileBuffers() when writing executables on Windows

This is a follow-up for "r325274: Call FlushFileBuffers on output files."

Previously, FlushFileBuffers() was called in all cases when writing a file. The objective was to go around a bug in the Windows kernel (as described here: https://randomascii.wordpress.com/2018/02/25/compiler-bug-linker-bug-windows-kernel-bug/). However that is required only when writing EXEs, any other file type doesn't need flushing.

This patch calls FlushFileBuffers() only for EXEs. In addition, we completly disable FlushFileBuffers() for known Windows 10 versions that do not exhibit the original kernel bug.

Differential Revision: https://reviews.llvm.org/D53727

llvm-svn: 346152