icu4c: 51.1

* Update hashes and drop old bottles.
* Enable build with Clang as [1] shouldn't be happening again.

[1] https://github.com/mxcl/homebrew/issues/16142

Closes #19354.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
icu4c: 51.1 bottles