[clang][IFS] Fixing assert in clang interface stubs for enums, records, typedefs

The clang IFS ASTConsumer was asserting on enums, records (struct definitions in
C), and typedefs. All it needs to do is skip them because the stub just needs to
expose global object instances and functions.

Differential Revision: https://reviews.llvm.org/D68859

llvm-svn: 374573