[Basic] Limit `version::getCompilerversion` to major + minor only
At the moment the only user of this method is `-interface-compiler-version`
and it's checked against major + minor only, so this is not going
to affect functionality.
In the future we should switch from `llvm::VersionTuple` to `swift::Version`
because swift tags have five components.
Resolves: rdar://140006577
