Add Windows application manifest to rustc-main
Windows allows setting some runtime options using a manifest file. The format of the XML file is documented here: https://docs.microsoft.com/en-us/windows/win32/sbscs/application-manifests
The manifest file in this PR does three things:
* Declares which Windows versions we support. This may help avoid unnecessary compatibility shims.
* Uses the UTF-8 code page. While Rust itself uses UTF-16 APIs, other code may rely on the code page.
* Makes the application long path aware (if also enabled by the user). This allows for the current directory to be longer than `PATH_MAX`.
These changes only affect the `rustc` process and not any other DLLs or compiled programs.
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