Use correct start location for class/function clause header

[dhruvmanila]

Summary

This PR fixes the bug where the formatter would panic if a class/function with
decorators had a suppression comment.

The fix is to use to correct start location to find the async/def/class
keyword when decorators are present which is the end of the last decorator.

Test Plan

Add test cases for the fix and update the snapshots.

[dhruvmanila]

Current dependencies on/for this PR:

• main
  • PR Use correct start location for class/function clause header #7802 👈

This comment was auto-generated by Graphite.

[codspeed-hq]

CodSpeed Performance Report

Merging #7802 will degrade performances by 2.37%

_{Comparing dhruv/start-location (b79b73d) with main (7b4fb4f)}

Summary

⚡ 1 improvements
❌ 1 regressions
✅ 23 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	dhruv/start-location	Change
⚡	linter/default-rules[pydantic/types.py]	38.9 ms	38 ms	+2.48%
❌	linter/all-rules[numpy/ctypeslib.py]	34.6 ms	35.5 ms	-2.37%

[konstin]

What about fmt_skip/decorators.py for the tests?