[lldb][NFC] Cleanup ClangASTContext::CompleteTagDeclarationDefinition

Makes this function exit early instead of nesting if statements.

Also removed all the if (tag_type->getDecl()) checks. If we created
a TagType with a nullptr as a Decl then Clang would have already
deferenced that nullptr during TagType creation so there is no point
in gracefully handling a nullptr here.