ast-exporter: Accept Float16 like Half

Float16 builtins appear as a consequence of converting RISCV vector
types to normal vector types.

Closes: #692

Author: chrysn

c2rust-ast-exporter/src/AstExporter.cpp

@@ -400,6 +400,9 @@ class TypeEncoder final : public TypeVisitor<TypeEncoder> {
             case BuiltinType::UInt: return TagUInt;
             case BuiltinType::ULong: return TagULong;
             case BuiltinType::ULongLong: return TagULongLong;
+            // Constructed as consequence of the conversion of built-in vector
+            // types to normal vector types
+            case BuiltinType::Float16: return TagHalf;
             case BuiltinType::Half: return TagHalf;     
             #if CLANG_VERSION_MAJOR >= 11
             case BuiltinType::BFloat16: return TagBFloat16;