[OpenCL] Fix layering violation by getOpenCLTypeAddrSpace

Commit 7ac28eb0 / r310911 ("[OpenCL] Allow targets to select address
space per type", 2017-08-15) made Basic depend on AST, introducing a
circular dependency.  Break this dependency by adding the
OpenCLTypeKind enum in Basic and map from AST types to this enum in
ASTContext.

Differential Revision: https://reviews.llvm.org/D40838


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319883 91177308-0d34-0410-b5e6-96231b3b80d8