Rename EH related stuff to be more precise

Summary:
The current "WinEH" exception handling type is more about Itanium-style
LSDA tables layered on top of the Windows native unwind info format
instead of .eh_frame tables or EHABI unwind info. Use the name
"ItaniumWinEH" to better reflect the hybrid nature of the design.

Also rename isExceptionHandlingDWARF to usesItaniumLSDAForExceptions,
since the LSDA is part of the Itanium C++ ABI document, and not the
DWARF standard.

Reviewers: echristo

Subscribers: llvm-commits, compnerd

Differential Revision: http://reviews.llvm.org/D6279

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222062 91177308-0d34-0410-b5e6-96231b3b80d8
DI: Use Metadata for DITypeRef and DIScopeRef

Now that `MDString` and `MDNode` have a common base class, use it.  Note
that it's not useful to assume subclasses of `Metadata` must be one or
the other since we'll be adding more subclasses soon enough.

Part of PR21532.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222064 91177308-0d34-0410-b5e6-96231b3b80d8
DIBuilder: Cleanup access control style, NFC

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222066 91177308-0d34-0410-b5e6-96231b3b80d8
IR: Remove MDString logic for Value::hasName()

This isn't necessary after r221960.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222067 91177308-0d34-0410-b5e6-96231b3b80d8
DIBuilder: Change private helper function to static, NFC

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222068 91177308-0d34-0410-b5e6-96231b3b80d8
DIBuilder: Use Constant instead of Value

Make explicit the requirement that most IR values in `DIBuilder` are
`Constant`.  This requires a follow-up change in clang.

Part of PR21532.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222070 91177308-0d34-0410-b5e6-96231b3b80d8