[-Wunsafe-buffer-usage] Cherrypick fix of FN warning when constructing a span from a C'tor member initializer (#91991)

[ziqingluo-90]

Respect the [[clang::unsafe_buffer_usage]] attribute for field and constructor initializers (llvm#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 llvm#80482

rdar://137999201
(cherry picked from commit a518ed2)

[ziqingluo-90]

@swift-ci test

[ziqingluo-90]

Still can't add @jkorous-apple to reviewer.

[ziqingluo-90]

I plan to go ahead and merge this cherry-pick tomorrow, if no objections.