[clang] [X86][Clang] Support constexpr evaluation of cvtpd2ps intrinsics (PR #169980)

[Simon Pilgrim via cfe-commits]

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@@ -0,0 +1,479 @@
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown -target-feature +avx -target-feature +avx512f -target-feature +avx512vl -verify %s
+
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RKSimon wrote:

It might be better to split these into 4 files:
```
constexpr-x86-sse2-builtins.cpp
constexpr-x86-avx-builtins.cpp
constexpr-x86-avx512f-builtins.cpp
constexpr-x86-avx512vl-builtins.cpp
```
as I expect more intrinsics will follow and we need to ensure we test using the correct target isa.

@tbaederr might have some suggestions on test styling.

https://github.com/llvm/llvm-project/pull/169980