…d variables (llvm#792)

In OpenCL, `local`-qualified variables are implicitly considered as
static. In order to support it, this PR unblocks code paths related to
OpenCL static declarations in `CIRGenDecl.cpp`.

Following the approach of LLVM CodeGen, a new class
`CIRGenOpenCLRuntime` is added to handle the language hook of creating
`local`-qualified variables. The default behavior of this hook is quite
simple. It forwards the call to `CGF.buildStaticVarDecl`.

So in CIR, the OpenCL local memory representation is equivalent to the
one defined by SPIR-LLVM convention: a `cir.global` with
`addrspace(local)` and *without initializer*, which corresponds to LLVM
`undef` initializer. See check lines in test for more details.

A `static global`-qualified variable is also added in the test to
exercise the static code path itself.