[llvm-ar] Improve MRI script CREATE command handling

I discovered that when compared to GNU the llvm-ar MRI script parsing of
CREATE could lead to some strange behaviour. This fix improves the error
message in the case when no archive name is given and will not allow the
adding of members until CREATE is called. Along with this change I added
more testing of the CREATE command.

Differential Revision: https://reviews.llvm.org/D128055