[compiler-rt] On Windows, silence warning when building with latest MSVC

This fixes:
```
[3334/7449] Building C object projects\compiler-rt\lib\builtins\CMakeFiles\clang_rt.builtins-x86_64.dir\extendsfdf2.c.obj
C:\git\llvm-project\compiler-rt\lib\builtins\fp_extend_impl.inc(63): warning C4018: '<': signed/unsigned mismatch
```

Author: aganea

compiler-rt/lib/builtins/fp_extend_impl.inc

@@ -60,7 +60,7 @@ static __inline dst_t __extendXfYf2__(src_t a) {
   dst_rep_t dstExp;
   dst_rep_t dstSigFrac;
 
-  if (srcExp >= 1 && srcExp < srcInfExp) {
+  if (srcExp >= 1 && srcExp < (src_rep_t)srcInfExp) {
     // a is a normal number.
     dstExp = (dst_rep_t)srcExp + (dst_rep_t)(dstExpBias - srcExpBias);
     dstSigFrac = (dst_rep_t)srcSigFrac << (dstSigFracBits - srcSigFracBits);