[OCaml] Fix undefined reference to LLVMDumpType() with NDEBUG

Account for the possibility of LLVMDumpType() not being available with
NDEBUG in the OCaml bindings. If it is not built into LLVM, make
the dump function raise an exception.

Since rL293359, the dump functions are built only if either NDEBUG is
not defined, or LLVM_ENABLE_DUMP is defined. As a result, if the dump
functions are not built in LLVM, the dynamic OCaml libraries fail to
load due to undefined LLVMDumpType symbol.

Differential Revision: https://reviews.llvm.org/D35899

llvm-svn: 309321