Partially reverts commit 1624354f33 which
moved a bunch of stuff from strings to enums. The issue here is that
Compiler.mode is not just, or primarily, something we compare, but is
instead written in as e.g. `rule c_{compiler.mode}` to build.ninja, so
this identifier needs to be a string.
Ultimately, the issue is that the commit tried to rewrite a bunch of
things called "mode" that had a couple of TODOs saying to use enums...
but it rewrote everything called "mode" regardless of whether it was a
function kwarg or a compiler property, even though the TODO only applied
to one of them.
34ac2e4af6