[AST] Rename several DeclContext methods to be clearer and shorter (#18798)

- getAsDeclOrDeclExtensionContext -> getAsDecl

This is basically the same as a dyn_cast, so it should use a 'getAs'
name like TypeBase does.

- getAsNominalTypeOrNominalTypeExtensionContext -> getSelfNominalTypeDecl
- getAsClassOrClassExtensionContext -> getSelfClassDecl
- getAsEnumOrEnumExtensionContext -> getSelfEnumDecl
- getAsStructOrStructExtensionContext -> getSelfStructDecl
- getAsProtocolOrProtocolExtensionContext -> getSelfProtocolDecl
- getAsTypeOrTypeExtensionContext -> getSelfTypeDecl (private)

These do /not/ return some form of 'this'; instead, they get the
extended types when 'this' is an extension. They started off life with
'is' names, which makes sense, but changed to this at some point.  The
names I went with match up with getSelfInterfaceType and
getSelfTypeInContext, even though strictly speaking they're closer to
what getDeclaredInterfaceType does. But it didn't seem right to claim
that an extension "declares" the ClassDecl here.

- getAsProtocolExtensionContext -> getExtendedProtocolDecl

Like the above, this didn't return the ExtensionDecl; it returned its
extended type.

This entire commit is a mechanical change: find-and-replace, followed
by manual reformatted but no code changes.

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -479,7 +479,7 @@ void walkRelatedDecls(const ValueDecl *VD, const FnTy &Fn) {
   }
 
   // Now provide the results along with whether the name is duplicate or not.
-  ValueDecl *OriginalBase = VD->getDeclContext()->getAsNominalTypeOrNominalTypeExtensionContext();
+  ValueDecl *OriginalBase = VD->getDeclContext()->getSelfNominalTypeDecl();
   for (auto Related : RelatedDecls) {
     ValueDecl *RelatedVD = Related.getValueDecl();
     bool SameBase = Related.getBaseDecl() && Related.getBaseDecl() == OriginalBase;