[CSSPGO] Fix MSVC initializing truncation warning (NFC)

MSVC warning:
```
\llvm-project\llvm\include\llvm\Transforms\IPO\SampleProfileProbe.h(65): warning C4305: 'initializing': truncation from 'double' to 'const float'
```

include/llvm/Transforms/IPO/SampleProfileProbe.h

@@ -62,7 +62,7 @@ public:
 
 private:
   // Allow a little bias due the rounding to integral factors.
-  constexpr static float DistributionFactorVariance = 0.02;
+  constexpr static float DistributionFactorVariance = 0.02f;
   // Distribution factors from last pass.
   FuncProbeFactorMap FunctionProbeFactors;