[Clang][Driver][SPIRV] Support LTO through the llvm-lto tool (#182347) There is no SPIR-V linker that supports LTO and a proposal to support basic SPIR-V linking in `lld` was [rejected](https://github.com/llvm/llvm-project/pull/178749), so support a basic version of LTO just by calling `llvm-lto` directly from the SPIR-V Toolchain. `-Xlinker` can be used to specify flags to `llvm-lto`. This should be enough for our use case. There is also the `llvm-lto2` tool, but that requires a list of symbol resolutions in the command line, and we can't compute that in the driver. --------- Signed-off-by: Nick Sarnie <nick.sarnie@intel.com>
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