[compiler-rt] Fix an MSVC build issue. (PR #111557)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

Silently skipping compiling bits of compiler-rt seems bad... it seems likely to lead to a situation where a toolchain is broken in a very subtle way.  These particular bits of compiler-rt probably aren't that important in most cases, but still, it seems problematic.  Could you explain why you're building compiler-rt with MSVC, instead of doing the usual LLVM_ENABLE_RUNTIMES build?

https://github.com/llvm/llvm-project/pull/111557