[clang] [clang] Replace "can't" and "can not" in diagnostics with "cannot" (PR #116623)

[Krzysztof Parzyszek via cfe-commits]

kparzysz wrote:

I (visually) looked at all occurrences of `can\>` in the td files, and I didn't see any occurring at the end of a line, or in a place where a `not` could be pasted into the message following the `can`.  It was a "best effort" inspection, but there weren't too many of these, so I'm pretty sure I got all of them.

https://github.com/llvm/llvm-project/pull/116623