Fix bugs in --clang-macro-fallback #2727

Job	Run time
docs	16s
minimal	10s
check-cfg	20s
msrv	50s
quickchecking	15s
rustfmt-clippy	30s
test-book	15s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 38s
test-expectations (ubuntu-latest)	38s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 40s
test-no-headers	31s
test-expectations (macos-latest)	42s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 39s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 34s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	2m 15s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	2m 13s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	2m 15s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	2m 13s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	2m 9s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	2m 16s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	2m 15s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	2m 14s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	2m 16s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	2m 14s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	2m 15s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	2m 17s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 18s
test (macos-latest, 16.0, 0, 0, 0)	2m 0s
success	1s
	42m 9s

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