Issue Description:

This string does not get concatenated:

$ cat testStr.swift
let sample = "a" + "გ" + "x"

$ /Library/Developer/Toolchains/swift-LOCAL-2017-03-22-a-base.xctoolchain/usr/bin/swiftc -Onone -emit-sil testStr.swift

bb0(%0 : $Int32, %1 : $UnsafeMutablePointer<Optional<UnsafeMutablePointer<Int8>>>):
alloc_global @_T07testStr6sampleSSv, loc "testStr.swift":1:5, scope 1 // id: %2
%3 = global_addr @_T07testStr6sampleSSv : $*String, loc "testStr.swift":1:5, scope 1 // user: %28
// function_ref static String.+ infix(String, String) -> String
%4 = function_ref @_T0SS1poiS2S_SStFZ : $@convention(method) (@owned String, @owned String, @thin String.Type) -> @owned String, loc "testStr.swift":1:26, scope 1 // user: %27
%5 = metatype $@thin String.Type, loc "testStr.swift":1:26, scope 1 // user: %27
// function_ref static String.+ infix(String, String) -> String
%6 = function_ref @_T0SS1poiS2S_SStFZ : $@convention(method) (@owned String, @owned String, @thin String.Type) -> @owned String, loc "testStr.swift":1:18, scope 1 // user: %20
%7 = metatype $@thin String.Type, loc "testStr.swift":1:18, scope 1 // user: %20
%8 = string_literal utf8 "a", loc "testStr.swift":1:14, scope 1 // user: %13
%9 = integer_literal $Builtin.Word, 1, loc "testStr.swift":1:14, scope 1 // user: %13
%10 = integer_literal $Builtin.Int1, -1, loc "testStr.swift":1:14, scope 1 // user: %13
%11 = metatype $@thin String.Type, loc "testStr.swift":1:14, scope 1 // user: %13
// function_ref String.init(_builtinStringLiteral : Builtin.RawPointer, utf8CodeUnitCount : Builtin.Word, isASCII : Builtin.Int1) -> String
%12 = function_ref @_T0S2SBp21_builtinStringLiteral_Bw17utf8CodeUnitCountBi1_7isASCIItcfC : $@convention(method) (Builtin.RawPointer, Builtin.Word, Builtin.Int1, @thin String.Type) -> @owned String, loc "testStr.swift":1:14, scope 1 // user: %13
%13 = apply %12(%8, %9, %10, %11) : $@convention(method) (Builtin.RawPointer, Builtin.Word, Builtin.Int1, @thin String.Type) -> @owned String, loc "testStr.swift":1:14, scope 1 // user: %20
%14 = string_literal utf16 "გ", loc "testStr.swift":1:20, scope 1 // user: %19
%15 = integer_literal $Builtin.Word, 1, loc "testStr.swift":1:20, scope 1 // user: %19
%16 = integer_literal $Builtin.Int1, 0, loc "testStr.swift":1:20, scope 1
%17 = metatype $@thin String.Type, loc "testStr.swift":1:20, scope 1 // user: %19
// function_ref String.init(_builtinUTF16StringLiteral : Builtin.RawPointer, utf16CodeUnitCount : Builtin.Word) -> String
%18 = function_ref @_T0S2SBp26_builtinUTF16StringLiteral_Bw18utf16CodeUnitCounttcfC : $@convention(method) (Builtin.RawPointer, Builtin.Word, @thin String.Type) -> @owned String, loc "testStr.swift":1:20, scope 1 // user: %19
%19 = apply %18(%14, %15, %17) : $@convention(method) (Builtin.RawPointer, Builtin.Word, @thin String.Type) -> @owned String, loc "testStr.swift":1:20, scope 1 // user: %20
%20 = apply %6(%13, %19, %7) : $@convention(method) (@owned String, @owned String, @thin String.Type) -> @owned String, loc "testStr.swift":1:18, scope 1 // user: %27
%21 = string_literal utf8 "x", loc "testStr.swift":1:28, scope 1 // user: %26
%22 = integer_literal $Builtin.Word, 1, loc "testStr.swift":1:28, scope 1 // user: %26
%23 = integer_literal $Builtin.Int1, -1, loc "testStr.swift":1:28, scope 1 // user: %26
%24 = metatype $@thin String.Type, loc "testStr.swift":1:28, scope 1 // user: %26
// function_ref String.init(_builtinStringLiteral : Builtin.RawPointer, utf8CodeUnitCount : Builtin.Word, isASCII : Builtin.Int1) -> String
%25 = function_ref @_T0S2SBp21_builtinStringLiteral_Bw17utf8CodeUnitCountBi1_7isASCIItcfC : $@convention(method) (Builtin.RawPointer, Builtin.Word, Builtin.Int1, @thin String.Type) -> @owned String, loc "testStr.swift":1:28, scope 1 // user: %26
%26 = apply %25(%21, %22, %23, %24) : $@convention(method) (Builtin.RawPointer, Builtin.Word, Builtin.Int1, @thin String.Type) -> @owned String, loc "testStr.swift":1:28, scope 1 // user: %27
%27 = apply %4(%20, %26, %5) : $@convention(method) (@owned String, @owned String, @thin String.Type) -> @owned String, loc "testStr.swift":1:26, scope 1 // user: %28
store %27 to %3 : $*String, loc "testStr.swift":1:26, scope 1 // id: %28
%29 = integer_literal $Builtin.Int32, 0, scope 1 // user: %30
%30 = struct $Int32 (%29 : $Builtin.Int32), scope 1 // user: %31
return %30 : $Int32, scope 1 // id: %31
} // end sil function 'main'