Convert clang::LangAS to a strongly typed enum

Convert clang::LangAS to a strongly typed enum

Summary:
Convert clang::LangAS to a strongly typed enum

Currently both clang AST address spaces and target specific address spaces
are represented as unsigned which can lead to subtle errors if the wrong
type is passed. It is especially confusing in the CodeGen files as it is
not possible to see what kind of address space should be passed to a
function without looking at the implementation.
I originally made this change for our LLVM fork for the CHERI architecture
where we make extensive use of address spaces to differentiate between
capabilities and pointers. When merging the upstream changes I usually
run into some test failures or runtime crashes because the wrong kind of
address space is passed to a function. By converting the LangAS enum to a
C++11 we can catch these errors at compile time. Additionally, it is now
obvious from the function signature which kind of address space it expects.

I found the following errors while writing this patch:

- ItaniumRecordLayoutBuilder::LayoutField was passing a clang AST address
  space to  TargetInfo::getPointer{Width,Align}()
- TypePrinter::printAttributedAfter() prints the numeric value of the
  clang AST address space instead of the target address space.
  However, this code is not used so I kept the current behaviour
- initializeForBlockHeader() in CGBlocks.cpp was passing
  LangAS::opencl_generic to TargetInfo::getPointer{Width,Align}()
- CodeGenFunction::EmitBlockLiteral() was passing a AST address space to
  TargetInfo::getPointerWidth()
- CGOpenMPRuntimeNVPTX::translateParameter() passed a target address space
  to Qualifiers::addAddressSpace()
- CGOpenMPRuntimeNVPTX::getParameterAddress() was using
  llvm::Type::getPointerTo() with a AST address space
- clang_getAddressSpace() returns either a LangAS or a target address
  space. As this is exposed to C I have kept the current behaviour and
  added a comment stating that it is probably not correct.

Other than this the patch should not cause any functional changes.

Reviewers: yaxunl, pcc, bader

Reviewed By: yaxunl, bader

Subscribers: jlebar, jholewinski, nhaehnle, Anastasia, cfe-commits

Differential Revision: https://reviews.llvm.org/D38816

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315871 91177308-0d34-0410-b5e6-96231b3b80d8