Implement atomic_*_explicit (e.g. atomic_store_explicit) with
memory_order plus optional memory_scope.
OpenCL memory_order maps 1:1 to Clang (e.g. OpenCL memory_order_relaxed
== Clang __ATOMIC_RELAXED), so we pass it unchanged to clc_atomic_*
function which forwards to Clang _scoped_atomic* builtins.
Other changes:
* Add __opencl_get_clang_memory_scope helper in opencl/utils.h (OpenCL
scope -> Clang scope).
* Correct atomic_compare_exchange return type to bool.
* Fix atomic_compare_exchange to return true when value stored in the
pointer equals expected value.
* Remove volatile from CLC functions so that volatile isn't present in
LLVM IR.
* Add '-fdeclare-opencl-builtins -finclude-default-header' flag to
include
declaration of memory_scope. Some constants in libclc are already
provided
by Clang’s OpenCL header; disable those in OpenCL library build and
enable them only for CLC library build.
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