[lld] [lldb] [clang-tools-extra] [libunwind] [flang] [llvm] [clang] [libc] [libcxx] [compiler-rt] [Clang] Add __builtin_vectorelements to get number of elements in vector (PR #69010)

[Lawrence Benson via cfe-commits]

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@@ -5126,6 +5126,14 @@ void CXXNameMangler::mangleExpression(const Expr *E, unsigned Arity,
       Diags.Report(DiagID);
       return;
     }
+    case UETT_VectorElements: {
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lawben wrote:

@philnik777 I'm adding you here because of your recent commit that adds `__datasizeof` (#67805). This is essentially the same code path and you also specify the "cannot mangle" for Itanium but not for Microsoft. Do you know if this is needed for Microsoft (see comments above)? 

I'm happy to address this for `__builtin_vectorelements`, but I really don't know where to start and `__datasizeof` seems to be the same.

https://github.com/llvm/llvm-project/pull/69010