[OpenCL] Fix layering violation by getOpenCLTypeAddrSpace

Commit 7ac28eb0a5 / 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

llvm-svn: 319883