Handle ObjCEncodeExpr in extractStringLiteralCharacter.

This fixes an assertion failure that occurs later in the function when
an ObjCEncodeExpr is cast to StringLiteral.

rdar://problem/30111207

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293596 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 3aa1edef05b0ae11b9c2f27a24644e35f0bf72b7)
2 files changed