[clang] [HIP][SPIRV] Enable the SPIRV backend instead of the translator through an experimental flag. (PR #162282)

[Alex Voicu via cfe-commits]

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+// This test case validates the behavior of -use-experimental-spirv-backend
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+// Test that -use-experimental-spirv-backend calls clang -cc1 with the SPIRV triple.
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AlexVlx wrote:

We need special handling because we do not invoke the BE in its default state, with its default flags. The vanilla target working "as you would expect" is not quite surprising, given that it's meant to work with said defaults.

https://github.com/llvm/llvm-project/pull/162282