Source mapping for class constructors should contain the class name

Testcase: https://polite-mochi-ddca0f.netlify.app/index.min.html
(Repo: https://github.com/mstange/profiler-sourcemap-testcase-constructor-name )

I get this source-mapped profile, showing time spent in a function named `constructor`: https://share.firefox.dev/4ft1z5a

In the original source code, this is the constructor of the `SlowThingy` class.

Can we get the `SlowThingy` name into the constructor name?

If the class name hasn't been minified, the pre-sourcemapping name of the function is just the class name. And then source mapping removes the class name and substitutes it with the word "constructor".

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-1388)

Start with the linked minified testcase and source-mapped profile to reproduce the constructor name shown as `constructor`. Trace the profiler's source-mapping handling for class constructors; done means a constructor from the `SlowThingy` example is displayed with the `SlowThingy` class name without regressing other mapped function names.

Written by the indexing model from the issue text.