[PR52549][clang-cl] Predefine _MSVC_EXECUTION_CHARACTER_SET

Since VS 2022 17.1 MSVC predefines _MSVC_EXECUTION_CHARACTER_SET to inform the users of the execution character set defined at compile time. The value the macro expands to is a Windows Code Page Identifier which are documented here: https://docs.microsoft.com/en-us/windows/win32/intl/code-page-identifiers

As clang currently only supports UTF-8 it is defined as 65001. If clang-cl were to support a different execution character set in the future we'd have to change the value.

Fixes https://bugs.llvm.org/show_bug.cgi?id=52549

Differential Revision: https://reviews.llvm.org/D114576