[Support] Add formatv support for StringLiteral

Summary:
This is achieved by generalizing the expression selecting the StringRef
format_provider. Now, anything that can be converted to a StringRef will
use it's formatter.

Reviewers: zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29898

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295064 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed