[OCaml] Pass -D/-UNDEBUG through to ocamlc

Detect [/-][DU]NDEBUG in CMAKE_C_FLAGS* and pass them through to ocamlc.
This is necessary because their value might affect visibility of dump
functions in LLVM and ocamlc uses its own compiler and flags by default.

Differential Revision: https://reviews.llvm.org/D35898

llvm-svn: 309483