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.

compiler-rt/lib/builtins/int_types.h

@@ -223,7 +223,7 @@ typedef union {
 #define CRT_HAS_TF_MODE
 #endif
 
-#if __STDC_VERSION__ >= 199901L
+#if __STDC_VERSION__ >= 199901L && !defined(_MSC_VER)
 typedef float _Complex Fcomplex;
 typedef double _Complex Dcomplex;
 typedef long double _Complex Lcomplex;