Add flag "/utf-8" for MSVC compiler in the build script

This will solve https://github.com/llvm/llvm-project/issues/60549.

> You need to add `-DCMAKE_CXX_CLAGS="/utf-8" to command line argument.
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> I got similar result with LLVM 18.1.8 and Visual Studio 19 in Japanese environment(CP932).
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> MSVC opens source file with system code page by default. 
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