Improve performance of `SourceMapper` #3106

Job	Run time
Coding Guidelines	32s
Dependency Validation	7s
Static Analysis	33s
Unit Tests (ubuntu-latest, 8.3)	17s
Unit Tests (ubuntu-latest, 8.4)	15s
Unit Tests (ubuntu-latest, 8.5)	11s
Unit Tests (windows-latest, 8.3)	32s
Unit Tests (windows-latest, 8.4)	30s
Unit Tests (windows-latest, 8.5)	31s
End-to-End Tests (ubuntu-latest, 8.3)	40s
End-to-End Tests (ubuntu-latest, 8.4)	41s
End-to-End Tests (ubuntu-latest, 8.5)	40s
End-to-End Tests (windows-latest, 8.3)	1m 46s
End-to-End Tests (windows-latest, 8.4)	1m 59s
End-to-End Tests (windows-latest, 8.5)	1m 43s
Build PHAR	37s
Code Coverage	2m 33s
Test PHAR (8.3, pcov)	18s
Test PHAR (8.3, xdebug)	32s
Test PHAR (8.4, pcov)	21s
Test PHAR (8.4, xdebug)	39s
Test PHAR (8.5, pcov)	30s
Test PHAR (8.5, xdebug)	31s
	16m 58s

Provide feedback