In well formed LLVM IR each Instruction is embedded in a BasicBlock. You can get the BasicBlock from getParent(). getParent() will always go one step up in the LLVM IR hierarchy, i.e., you get a Function as parent from a BasicBlock, and the Module from a Function.

The DeclContext getParent method returns the "semantic" parent. Although the concept of semantic parent has some quirks (see the getLookupParent method implementation), to a first approximation, the semantic parent of B is A if B can be found by qualified lookup as A::B.

But the code in question is being interpreted as C, not C++ (per a clarification in a comment), and C does not have qualified lookup at all! C does allow structure definitions to appear inside other structure definitions, for example:

struct Outer {
  struct Inner {
    int i;
  };
  int o;
};

but the effect is semantically the same as if they had not been nested:

struct Outer {
  int o;
};
struct Inner {
  int i;
};

In particular, to name the inner type in C, one writes just struct Inner, not struct Outer::Inner. See C99 6.7.2.1p7 and compare to C++11 9.2p1. See also the question Nested structures in C and C++.

Consequently, getParent returns the TranslationUnitDecl for both RecordDecls.

As you've noted, you can instead use the getLexicalParent method to get the syntactic context in which the declaration appeared.

If you compile the code as C++, either by changing the file extension to one that clang recognizes as being C++ (such as .cc or .cpp), or pass the -xc++ command line option to the command line parser, then the semantic parent of Inner will be reported as Outer.