[llvm-c] Added a C-API binding to query the LLVM version

The LLVM C bindings currently offer no way to query the version string
dynamically. This is a useful feature in situations where a program
isn't compiled against a specific version of LLVM but rather loads it
dynamically (e.g. using dlopen()).

In situations where the shared library filename doesn't reveal the
version (e.g. LLVM-C.dll) and to adapt to version-specific API
differences, it is then useful to be able to query the version string by
calling the proposed LLVMGetVersion function.

Differential Revision: https://reviews.llvm.org/D139381