[llvm] [NFC][ADT] add NumDigitsBase10 to MathExtra and update some current users (PR #165479)

[Timm Baeder via llvm-commits]

tbaederr wrote:

There is also https://github.com/llvm/llvm-project/blob/028bfa255e90581d1c08237a66c20b25096277e8/clang/lib/Frontend/TextDiagnostic.cpp#L1062-L1068 which _seems_ like a much simpler implementation.

https://github.com/llvm/llvm-project/pull/165479