[llvm] [Support] Always call FlushFileBuffers() when unmapping memory on Windows (PR #78597)

[Tobias Hieta via llvm-commits]

tru wrote:

What would be the right way to test this? We produce PDB files bigger than 4GB so it's not a huge deal for me to test. 

https://github.com/llvm/llvm-project/pull/78597