Respect the [[clang::unsafe_buffer_usage]] attribute for field and constructor initializers (#91991)

CXXCtorInitializers are not statements , but they point to an
initializer expression which is. When visiting a FunctionDecl, also
walk through any constructor initializers and run the warning
checks/matchers against their initializer expressions. This catches
warnings for initializing fields and calling other constructors, such
as:
    
struct C {
  C(P* Ptr) : AnUnsafeCtor(Ptr) {}
}

Field initializers can be found by traversing CXXDefaultInitExprs. This
catches warnings in places such as:
    
struct C {
  P* Ptr;
  AnUnsafeCtor U{Ptr};
};

We add tests for explicit construction, for field initialization, base
class constructor calls, delegated constructor calls, and aggregate
initialization.

Note that aggregate initialization is not fully covered where a field
specifies an initializer and it's not overridden in the aggregate initialization,
such as in:

struct AggregateViaValueInit {
    UnsafeMembers f1;
    // FIXME: A construction of this class does initialize the field
    // through this initializer, so it should warn. Ideally it should
    // also point to where the site of the construction is in
    // testAggregateViaValueInit().
    UnsafeMembers f2{3};
};

void testAggregateViaValueInit() {
    auto A = AggregateViaValueInit();
};

There are 3 tests for different types of aggregate initialization with
FIXMEs documenting this future work.

One attempt to fix this involved returning true from
MatchDescendantVisitor::shouldVisitImplicitCode(), however, it breaks expectations
for field in-class initializers by moving the SourceLocation, possibly
to inside the implicit ctor instead of on the line where the field
initialization happens.

struct C {
  P* Ptr;
  AnUnsafeCtor U{Ptr};  // expected-warning{{this is never seen then}}
};

Tests are also added for std::span(ptr, size) constructor being called
from a field initializer and a constructor initializer.

Issue #80482

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