[modules] Mark deleted functions as implicitly inline to allow merging

Summary: When merging definitions with ModulesLocalVisibility enabled it's important to make deleted definitions implicitly inline, otherwise they'll be diagnosed as a redefinition.

Reviewers: silvas, manmanren, rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D26143

llvm-svn: 285655