Fix bugs in --clang-macro-fallback #2968

Job	Run time
rustfmt-clippy	24s
msrv	21s
check-cfg	20s
minimal	11s
quickchecking	17s
docs	18s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 0s
test-book	27s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	49s
test-no-headers	20s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	57s
test-expectations (ubuntu-latest)	38s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	56s
test-expectations (macos-latest)	47s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	50s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	56s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	51s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	45s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	47s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	48s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	48s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	49s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	49s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	48s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	48s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	48s
test (ubuntu-latest, 16.0, 0, 0, 1)	48s
test (macos-latest, 16.0, 0, 0, 0)	47s
success	1s
	19m 8s

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