[clang-format] [PR43531] clang-format damages "alternative representations" for operators

Summary:
https://bugs.llvm.org/show_bug.cgi?id=43531

Fix for clang-format incorrectly handles "alternative operators" as described by https://en.cppreference.com/w/cpp/language/operator_alternative

compl = ~
not = !

these are unary operators, and clang-format will remove the space between them and a numeric constant

this incorrectly formats the following code

```
int a compl 5;
int a not 5;
```

into:

```
int a compl5;
int a not5;
```

The code adds FIXME unit tests for "alternative token" representations for {} [] and # as defined by the same link, which would require a more detailed change to the FormatTokenLexer

Reviewers: klimek, reuk, owenpan, mitchell-stellar, STL_MSFT

Reviewed By: mitchell-stellar

Subscribers: cfe-commits

Tags: #clang-format, #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D68332

llvm-svn: 373750