[Clang] Add __builtin_vectorelements to get number of elements in vector (#69010)

Adds a new `__builtin_vectorelements()` function which returns the
number of elements for a given vector either at compile-time for
fixed-sized vectors, e.g., created via `__attribute__((vector_size(N)))`
or at runtime via a call to `@llvm.vscale.i32()` for scalable vectors,
e.g., SVE or RISCV V.

The new builtin follows a similar path as `sizeof()`, as it essentially
does the same thing but for the number of elements in vector instead of
the number of bytes. This allows us to re-use a lot of the existing
logic to handle types etc.

A small side addition is `Type::isSizelessVectorType()`, which we need
to distinguish between sizeless vectors (SVE, RISCV V) and sizeless
types (WASM).

This is the [corresponding
discussion](https://discourse.llvm.org/t/new-builtin-function-to-get-number-of-lanes-in-simd-vectors/73911).

1 files changed, 3 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index cd7c26a..f3c55d0 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -8752,8 +8752,9 @@ ExprResult Sema::SemaConvertVectorExpr(Expr *E, TypeSourceInfo *TInfo,
                           diag::err_convertvector_non_vector)
                      << E->getSourceRange());
   if (!DstTy->isVectorType() && !DstTy->isDependentType())
-    return ExprError(Diag(BuiltinLoc,
-                          diag::err_convertvector_non_vector_type));
+    return ExprError(Diag(BuiltinLoc, diag::err_builtin_non_vector_type)
+                     << "second"
+                     << "__builtin_convertvector");
 
   if (!SrcTy->isDependentType() && !DstTy->isDependentType()) {
     unsigned SrcElts = SrcTy->castAs<VectorType>()->getNumElements();