Incorrect builtins are exposed for offload targets

[AaronBallman]

When using -aux-triple for an offload target, the incorrect builtins are being exposed. The builtins exposed are not based on the target specified. e.g,

static_assert(__has_builtin(__cpuidex), "uh oh");

will not fail when given: -c -fopenmp -Xclang -fopenmp-is-target-device -Xclang -aux-triple -Xclang x86_64-unknown-linux-gnu, but it should fail because that's a Windows-specific builtin. Removing the -aux-triple will cause the assertion to trigger as expected.

https://godbolt.org/z/sbMjjer16

[llvmbot]

@llvm/issue-subscribers-bug

[llvmbot]

@llvm/issue-subscribers-clang-frontend