[clang-format] Don't squash Verilog escaped identifiers

An escaped identifier always needs a space following it so the parser
can tell it apart from the next token.

The unit tests are changed to use `FormatTestBase.h` because we need the
2-argument version of `verifyFormat`.  We also added the `messUp`
virtual function because Verilog needs a different version of it.

Reviewed By: HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D146401