[clang-format] Link the braces of a block in UnwrappedLineParser

This includes TT_MacroBlockBegin and TT_MacroBlockEnd as well.

We can no longer use MatchingParen of FormatToken as an indicator
to mark optional braces. Instead, we directly set Optional of an
l_brace first and reset it later if it turns out that the braces
are not optional.

Also added a test case for deeply nested loops.

Differential Revision: https://reviews.llvm.org/D139257