[clang][test] fix typo in fn attr

While testing backports of
https://reviews.llvm.org/D129572#inline-1245936
commit 2240d72f ("[X86] initial -mfunction-return=thunk-extern support")
I noticed that one of my unit tests mistyped a function attribute. The
unit test was intended to test fn attr merging behavior, but with the
typo it was not.  Small fixup.

Reviewed By: aaron.ballman, erichkeane

Differential Revision: https://reviews.llvm.org/D129691