[clang] Mark __builtin_convertvector and __builtin_shufflevector as constexpr. (#112129)

Closes #107985.

LanguageExtensions.rst states that `__builtin_shufflevector` and
`__builtin_convertvector` can be evaluated as constants, but this is not
reflected in Butiltins.td. This patch aligns these two.

Author: c8ef

clang/include/clang/Basic/Builtins.td

@@ -1198,13 +1198,13 @@ def AllowRuntimeCheck : Builtin {
 
 def ShuffleVector : Builtin {
   let Spellings = ["__builtin_shufflevector"];
-  let Attributes = [NoThrow, Const, CustomTypeChecking];
+  let Attributes = [NoThrow, Const, CustomTypeChecking, Constexpr];
   let Prototype = "void(...)";
 }
 
 def ConvertVector : Builtin {
   let Spellings = ["__builtin_convertvector"];
-  let Attributes = [NoThrow, Const, CustomTypeChecking];
+  let Attributes = [NoThrow, Const, CustomTypeChecking, Constexpr];
   let Prototype = "void(...)";
 }
 

clang/test/Preprocessor/feature_tests.cpp

@@ -64,6 +64,14 @@
 #error Clang should have these constexpr builtins
 #endif
 
+#if !__has_constexpr_builtin(__builtin_convertvector)
+#error Clang should have these constexpr builtins
+#endif
+
+#if !__has_constexpr_builtin(__builtin_shufflevector)
+#error Clang should have these constexpr builtins
+#endif
+
 #if  __has_constexpr_builtin(__builtin_cbrt)
 #error This builtin should not be constexpr in Clang
 #endif