Make Windows objects reproducible so early cutoff holds therefeature/action-level-incremental

The Windows runner showed that a comment in a header recompiled both
units and then archived and relinked, where Linux and macOS stopped
after the compiles. Clang stamps objects for Windows targets with the
time by default, and debug information there records the content of
every header, so the objects always differed.

- Clang gets -mno-incremental-linker-compatible for Windows targets:
  the same source gives the same object.
- The early-cutoff part of BuildCommandTest builds in release mode,
  without debug information, on every system.

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