Fix bugs in --clang-macro-fallback #2969

Job	Run time
docs	20s
rustfmt-clippy	39s
minimal	15s
check-cfg	22s
msrv	1m 4s
quickchecking	17s
test-book	7s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 44s
test-expectations (ubuntu-latest)	42s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 35s
test-no-headers	26s
test-expectations (macos-latest)	1m 3s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 40s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 36s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	2m 3s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	2m 18s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	2m 10s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	2m 6s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	4m 12s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	4m 16s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	5m 42s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	4m 3s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	5m 13s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	4m 58s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	5m 45s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	5m 9s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 44s
test (macos-latest, 16.0, 0, 0, 0)	2m 17s
success	1s
	1h 4m 47s

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