[clang][ASTImporter] Only reorder fields of RecordDecls (#77079)

Prior to `e9536698`
(https://reviews.llvm.org/D154764) we only re-ordered the fields of
`RecordDecl`s. The change refactored this logic to make sure
`FieldDecl`s are imported before other member decls. However, this
change also widened the types of `DeclContext`s we consider for
re-ordering from `RecordDecl` to anything that's a `DeclContext`. This
seems to have been just a drive-by cleanup.

Internally we've seen numerous crashes in LLDB where we try to perform
this re-ordering on fields of `ObjCInterfaceDecl`s.

This patch restores old behaviour where we limit the re-ordering to just
`RecordDecl`s.

rdar://119343184
rdar://119636274
rdar://119832131