[clang-format] Properly handle the C11 _Generic keyword.

This patch properly recognizes the generic selection expression
introduced in C11, by adding an additional token type for the colons
present in such expressions.

Previously, they would be recognized as
"inline ASM colons" purely by the fact that those are the last thing
checked for.

I tried to avoid adding an addition token type, but since colons by
default like having spaces around them, I chose to add a new type so
that no space is added after the type selector.

Currently, no aspect of the formatting of these expressions in able to
be configured, as I'm not sure what could even be configured here.

One notable thing is that association list is always formatted as
either entirely on one line, if it can fit, or with line breaks
after every comma in the expression (also after the controlling expr.)

This visually makes them more similar to switch statements when long,
matching the behaviour of the selection expression, being that of a sort
of switch on types, but also allows for terseness when only selecting
for a few things.

Fixes https://github.com/llvm/llvm-project/issues/18080

Reviewed By: HazardyKnusperkeks, owenpan, MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D139211