[LLVM-C] Add Bindings For Module Flags

Summary:
The first foray into merging debug info into the echo tests.

- Add bindings to Module::getModuleFlagsMetadata() in the form of LLVMCopyModuleFlagsMetadata
- Add the opaque type LLVMModuleFlagEntry to represent Module::ModuleFlagEntry
- Add accessors for LLVMModuleFlagEntry's behavior, key, and metadata node.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: aprantl, JDevlieghere, llvm-commits, harlanhaskins

Differential Revision: https://reviews.llvm.org/D46792

llvm-svn: 332219