[clang] Introduce support for disabling warnings in system macros

Often we run into situations where we want to ignore
warnings from system headers, but Clang will still
give warnings about the contents of a macro defined
in a system header used in user-code.

Introduce a ShowInSystemMacro option to be able to
specify which warnings we do want to keep raising
warnings for. The current behavior is kept in this patch
(i.e. warnings from system macros are enabled by default).
The decision as to whether this should be an opt-in or opt-out
feature can be made in a separate patch.

To put the feature to test, replace duplicated code for
Wshadow and Wold-style-cast with the SuppressInSystemMacro tag.
Also disable the warning for C++20 designators, fixing #52944.

Differential Revision: https://reviews.llvm.org/D116833