[llvm] [llvm-objcopy][COFF] Update WinCFGuard section contents after stripping (PR #153322)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> It is still not a universal solution, as actually .symidx could be used in any section but WinCFGuard sections consist only of them.

Going through places where we use emitCOFFSymbolIndex() in LLVM, there's also `.gehcont$y` (for `/guard:ehcont`), `.impcall` (AArch64 import call optimization), and `.hybmp$x` (Arm64EC).

https://github.com/llvm/llvm-project/pull/153322