[libclc] [libclc] Use native spirv*-mesa-mesa3d clang triple for mesa (PR #199618)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> I don't understand why we are now adding the concept of triples to SPIR-V and GPU runtimes. And I really really really don't want that we imply in any way that applications would need to provide their OpenCL runtime specific SPIR-V binaries. Please make no assumptions about how FMA is implemented, no assumptions about denorm behavior or anything, because we'll have to support the same spirv across varies hardware that have different implementation details.

I'm increasingly confused by how you're using libclc. Do you have a description of your system written somewhere? End user code and implementation provider code are fundamentally different uses. You're using it as the compiler runtime library, but also don't want it to be target specific? These are contradictory. If libclc is providing the ultimate implementation, it's necessarily hardware specific? This is my point about decomposing the libraries. The target specific codegen bits ought to be separate from the user facing language entry points 

https://github.com/llvm/llvm-project/pull/199618