Issue #6419: Better clang version detection for Apple Clang

Apple Clang uses different version numbering than "regular" clang, but
it also provides the "regular" version it's based on. Update the sed
pattern to pull out this "regular" version number instead of the Apple
version number.
auto merge of #6739 : kballard/rust/issue-6419, r=catamorphism

Apple Clang uses different version numbering than "regular" clang, but
it also provides the "regular" version it's based on. Update the sed
pattern to pull out this "regular" version number instead of the Apple
version number.