Include RecordDecls from anonymous unions in the AST.

For

  void f() {
    union { int i; };
  }

clang used to omit the RecordDecl from the anonymous union from the AST.
That's because the code creating it only called PushOnScopeChains(), which adds
it to the current DeclContext, which here is the function's DeclContext. But
RecursiveASTVisitor doesn't descent into all decls in a FunctionDecl.

Instead, for DeclContexts that contain statements, return the RecordDecl so
that it can be included in the DeclStmt containing the VarDecl for the union.

Interesting bits from the AST before this change:

|-FunctionDecl
| `-CompoundStmt
|   |-DeclStmt
|   | `-VarDecl 0x589cd60 <col:3> col:3 implicit used 'union (anonymous at test.cc:3:3)' callinit

After this change:

-FunctionDecl
| `-CompoundStmt
|   |-DeclStmt
|   | |-CXXRecordDecl 0x4612e48 <col:3, col:18> col:3 union definition
|   | | |-FieldDecl 0x4612f70 <col:11, col:15> col:15 referenced i 'int'
|   | `-VarDecl 0x4613010 <col:3> col:3 implicit used 'union (anonymous at test.cc:3:3)' callinit

This is now closer to how anonymous struct and unions are represented as
members of structs.  It also enabled deleting some one-off code in the
template instantiation code.

Finally, it fixes a crash with ASTMatchers, see the included test case
(this fixes http://crbug.com/580749).

llvm-svn: 259079