[clang] [llvm] Enable fexec-charset option (PR #138895)

[Richard Smith via llvm-commits]

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+// RUN: %clang_cc1 %s -emit-llvm -triple s390x-none-zos -fexec-charset IBM-1047 -o - | FileCheck %s
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zygoloid wrote:

Should there be tests here for wide (`L`-prefixed) character and string literals too?

https://github.com/llvm/llvm-project/pull/138895