builtins: Speculative MSVC fix.
Attempt to fix these build failures:
https://lab.llvm.org/buildbot/#/builders/107/builds/12601
The suspected cause is that #133530 caused us to start
passing -std:c11 to MSVC, which activated this code path
that uses _Complex, which MSVC does not support. See:
https://learn.microsoft.com/en-us/cpp/c-runtime-library/complex-math-support
Fix it by also checking _MSC_VER.
