[LLVM-C] Fix name mangling on AggressiveInstCombine

Similarly to rL336736, at least one more C API function does not
properly get declared as extern "C" due to a missing header, causing
name mangling and linking errors.

This patch fixes calls to LLVMAddAggressiveInstCombinerPass().

Differential Revision: https://reviews.llvm.org/D49416

Reviewed By: whitequark

llvm-svn: 337264