review: add explicit comment of how the float is handled

Author: inbelic

clang/include/clang/Parse/ParseHLSLRootSignature.h

@@ -134,6 +134,14 @@ class RootSignatureParser {
   /// 32-bit integer
   std::optional<int32_t> handleIntLiteral(bool Negated);
   /// Use NumericLiteralParser to convert CurToken.NumSpelling into a float
+  ///
+  /// This matches the behaviour of DXC, which is as follows:
+  ///  - convert the spelling with `strtod`
+  ///  - check for a float overflow
+  ///  - cast the double to a float
+  /// The behaviour of `strtod` is replicated using:
+  ///  Semantics: llvm::APFloat::Semantics::S_IEEEdouble
+  ///  RoundingMode: llvm::RoundingMode::NearestTiesToEven
   std::optional<float> handleFloatLiteral(bool Negated);
 
   /// Flags may specify the value of '0' to denote that there should be no