DIGlobalVariable: Investigate representation of global variables

[langston-barrett]

This commit changed the representation of global variables in the bitcode: llvm-mirror/llvm@5420de3.

Some places in the reader now check for different versions of global-variable-involving expressions:

• https://github.com/llvm-mirror/llvm/blob/release_60/lib/Bitcode/Reader/MetadataLoader.cpp#L1540
• https://github.com/llvm-mirror/llvm/blob/release_40/lib/Bitcode/Reader/MetadataLoader.cpp#L1467

It seems the code in the parser is somewhat aware of this change:

• https://github.com/GaloisInc/llvm-pretty-bc-parser/blob/master/src/Data/LLVM/BitCode/IR/Metadata.hs#L652

I'm not sure exactly what the impact will be, though it does seem to affect the AST, changing the type of a field of DIGlobalVariable to be a DIExpression.