Merging r293596:

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r293596 | ahatanak | 2017-01-30 18:31:39 -0800 (Mon, 30 Jan 2017) | 7 lines

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

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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_40@293653 91177308-0d34-0410-b5e6-96231b3b80d8