[PERF] Don't spawn so many compilers (3/2) (19m -> 250k) (#15030)

Optimize `needless_doctest_main`, make it short-circuit, make sure that
we don't spin up a new compiler on EVERY code block.

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The old implementation was creating a new compiler, new parser, new
thread, new SessionGlobals, new everything for each code block. No
matter if they actually didn't even contain `fn main()` or anything
relevant.

On callgrind, seems that we're reducing about a 6.7242% de cycle count
(which turns out to be a 38 million instruction difference, great!).
Benchmarked in `bumpalo-3.16.0`. Also on bumpalo we spawn 78 less
threads. This moves `SessionGlobals::new` from the top time-consuming
function by itself in some benchmarks, into one not even in the top 500.

Also, populate the test files.

changelog:[`needless_doctest_main`]: Avoid spawning so many threads in
unnecessary circumstances

0 files changed, 0 insertions, 0 deletions