LLVM version check

Since llvm-spirv can now be used implicitly from clang I would like to suggest improving error detection related to interfacing of two separately installed tools due to incompatible versions. This could be done by adding a check in the translator for the LLVM version that each revision supports and if LLVM IR is detected of unsupported version an error can be reported but suppressed by the flag if necessary.

For example, we could check the version metadata of the compiling module. Any other suggestions are welcome.

CC @svenvh

Start in the translator and investigate how LLVM IR version metadata is currently exposed, along with the proposed suppression flag behavior. Define the supported-version check and error-reporting scope before implementing it, then add coverage for compatible, incompatible, and suppressed cases.

Written by the indexing model from the issue text.