The MSAN builder should use an MSAN instrument libcrack:
CURRENT_TEST: plugins.cracklib_password_check
mysqltest: At line 30: query 'grant select on . to foobar identified by 'q$%^&*rty'' failed: 2013: Lost connection to MySQL server during query
…
Uninitialized bytes in __interceptor_strchr at offset 8 inside [0x7fbc7d80c150, 9)
==17013==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x7fbc7cadff43 (/usr/lib/x86_64-linux-gnu/libcrack.so.2+0x3f43)
#1 0x7fbc7cae0356 in FascistCheckUser (/usr/lib/x86_64-linux-gnu/libcrack.so.2+0x4356)
#2 0x7fbc7cceb043 in crackme /buildbot/msan-ubuntu-1804-clang10/build/plugin/cracklib_password_check/cracklib_password_check.c:36:13