Unify the code that updates the ArchSpec after finding a fat binary

with how it is done for a lean binary

In particular this affects how target create --arch is handled — it
allowed us to override the deployment target (a useful feature for the
expression evaluator), but the fat binary case didn't.

rdar://problem/66024437

Differential Revision: https://reviews.llvm.org/D85049

(cherry picked from commit 470bdd3caaab0b6e0ffed4da304244be40b78668)