Some languages have the flexibility to use upper or lower case
characters interchangeably in integer and float literal definitions.
I'd like to be able to enforce a consistent case style in one of my
projects, so I added this clang-format style option to control it.
With this .clang-format configuration:
```yaml
NumericLiteralCaseStyle:
UpperCasePrefix: Never
UpperCaseHexDigit: Always
UpperCaseSuffix: Never
```
This line of code:
```C
unsigned long long 0XdEaDbEeFUll;
```
gets reformatted into this line of code:
```C
unsigned long long 0xDEAFBEEFull;
```
-----
I'm new to this project, so please let me know if I missed something in
the process. I modeled this PR from
[IntegerLiteralSeparatorFixer](https://reviews.llvm.org/D140543)
