[clang] [Clang] Set default LTO mode for AMDGCN/SPIR-V targets to full (PR #201457)

[Manuel Carrasco via cfe-commits]

mgcarrasco wrote:

@jhuber6 I think this PR might have broken the compilation to SPIRV for HIP (translator and backend). I reverted it locally and it solved at least the problems with the backend path. 

https://github.com/llvm/llvm-project/pull/201457