Crossplat MSBuild and assumptions about the file system

There are some places in MSBuild that restrict what characters a path may have, assume the FS is case insensitive, impose limits on path lengths, etc.
For example, [this code](https://github.com/Microsoft/msbuild/blob/xplat/src/XMakeBuildEngine/Utilities/EngineFileUtilities.cs#L88-L91) ignores the expansion of some globs with escaped `*` characters in them. Unix paths can have `*` in them.

We should probably at some point decide whether MSBuild should keep Windows only path constraints on crossplat. One argument for preserving the same constraints over all OSes is that building the same crossplat project on multiple OSes should result in the same build output.