[llvm] [formatv][FmtAlign] Use fill count of type size_t instead of uint32_t (PR #78459)

[Andrei Golubev via llvm-commits]

andrey-golubev wrote:

@zturner0826 @labath please take a look (you seem to be the ones who worked on this before)!

https://github.com/llvm/llvm-project/pull/78459