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

[Mike Hommey via cfe-commits]

glandium wrote:

This is also causing problems using the plugin API, because llvm/Support/TextEncoding.h includes llvm/Config/config.h which is not part of what's installed (which sounds it's a problem of its own, BTW, considering TextEncoding.h is part of what's installed).

(clang/Frontend/FrontendPluginRegistry.h includes clang/Frontend/FrontendAction.h includes clang/Frontend/ASTUnit.h includes clang/Frontend/PrecompiledPreamble.h includes clang/Lex/Preprocessor.h which this patches modified)

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