[SymbolGraphGen] synthesize child symbols for type aliases of private decls

[QuietMisdreavus]

Resolves rdar://141460819

Consider the following C code:

typedef struct {
  int myField;
} MyStruct;

typedef struct OtherStruct {
  int myField;
} OtherStruct;

When this C code is processed in ClangImporter, it initially sees four top-level Clang decls: two typedefs and their respective underlying struct decls, one anonymous and one named. In canSkipOverTypedef, it then folds the underlying type into the typedef decl, creating two final Swift top-level decls.

However, when processing this code:

typedef struct _MyStruct {
  int myField;
} MyStruct;

...this folding process fails, instead creating Swift decls for both the typedef and its underlying type.

When these decls are then rendered in SymbolGraphGen, the underlying struct decl is then hidden, due to its underscored name, leaving the converted type alias pointing to nowhere.

This PR adds behavior in SymbolGraphGen to effectively fold these decls together: When a type alias (including a converted Clang typedef) is encountered whose underlying type is (1) from the same module and (2) hidden by the current settings, it will crawl the underlying type and create synthesized symbols to clone its children as children of the type alias.

[QuietMisdreavus]

@swift-ci Please smoke test

[franklinsch]

Do we also need to support the other kinds of relationships like inheritsFrom, requirementOf, etc.?

[QuietMisdreavus]

I handled the other kinds of relationship that are necessary for type declarations in b7079f4 - this includes inheritsFrom and conformsTo, since requirementOf only applies to method declarations on protocols. Any other relationship kind that applies to the synthesized children will happen as these symbols are processed.

[QuietMisdreavus]

@swift-ci Please smoke test