FrozenClock: accepts DateTimeInterface as the initial time #58

Job	Run time
Static analysis (ubuntu-latest, 8.1)	25s
Coding standard (ubuntu-latest, 8.1)	16s
Tests (ubuntu-latest, 7.4, false)	22s
Test for mutants (ubuntu-latest, 8.1)	38s
Tests (ubuntu-latest, 8.0, false)	26s
Tests (ubuntu-latest, 8.1, false)	32s
Tests (macos-latest, 7.4, false)	1m 4s
Tests (macos-latest, 8.0, false)	1m 37s
Tests (macos-latest, 8.1, false)	1m 43s
Tests (windows-latest, 7.4, false)	1m 13s
Tests (windows-latest, 8.0, false)	1m 11s
Tests (windows-latest, 8.1, false)	1m 14s
Tests (ubuntu-latest, 7.4, --prefer-lowest --prefer-stable, false)	25s
Tests (ubuntu-latest, 8.2, --ignore-platform-req=php+, false)	25s
Status check - CI	2s
Code coverage finish (ubuntu-latest, 8.1)	2s
	11m 35s

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