[clang-format] Allow configuring list of macros that map to attributes

This adds a `AttributeMacros` configuration option that causes certain
identifiers to be parsed like a __attribute__((foo)) annotation.
This is motivated by our CHERI C/C++ fork which adds a __capability
qualifier for pointer/reference. Without this change clang-format parses
many type declarations as multiplications/bitwise-and instead.
I initially considered adding "__capability" as a new clang-format keyword,
but having a list of macros that should be treated as attributes is more
flexible since it can be used e.g. for static analyzer annotations or other language
extensions.

Example: std::vector<foo * __capability> -> std::vector<foo *__capability>

Depends on D86775 (to apply cleanly)

Reviewed By: MyDeveloperDay, jrtc27

Differential Revision: https://reviews.llvm.org/D86782