[llvm-ar] Add --thin for creating a thin archive

In GNU ar (since 2008), the modifier 'T' means creating a thin archive.
In many other ar implementations (FreeBSD, macOS, elfutils, etc), -T
means "allow filename truncation of extracted files", as specified by
X/Open System Interface.

For portability, 'T' with thin archive semantics should be avoided.

See https://sourceware.org/bugzilla/show_bug.cgi?id=28759 binutils 2.38
will deprecate 'T' (without diagnostic) and add --thin.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D116979