libclc support

This MR is based on top of !37 (merged) (support for complex types) and !29 (merged) (support for compile-time-constant data), as well as including several upstream cherry-picks to make reconciliation easier. It is based on mesa/mesa!4323 (closed) with fixups from the comments there, as well as reverting our own additions of broken implementations of OpenCL standard functions.

It adds support to link in a SPIR-V library produced by libclc, which is used as a lookaside: every time we encounter a call to an OpenCL built-in, we translate that into an OpenCL C standard function name, look for a matching function in the libclc library, and inline it there if so, as well as copying any required variables.

This is enough to get the existing tests (including cosh, which references an array of float vectors) working using the libclc built-ins, instead of our hand-hacked ones.