This patch adds a Clang-compatible `--save-stats` option, to provide an
easy to use way to save LLVM statistics files when working with llc on
the backend.
Like on Clang, one can specify `--save-stats`, `--save-stats=cwd`, and
`--save-stats=obj` with the same semantics and JSON format.
The implementation uses 2 methods `MaybeEnableStats` and
`MaybeSaveStats` called before and after `compileModule` respectively
that externally own the statistics related logic, while `compileModule`
is now required to return the resolved output filename via an output
param.
Note: like on Clang, the pre-existing `--stats` option is not affected.
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