Add DWARF calling convention to DISubroutineType

[langston-barrett]

It looks like the current implementation is based on the one in the 3.8 release. The one in the 6.0 release is significantly more complex. In particular, it looks like there are two significant changes that were made. One is #39, and the other is the addition of a field representing the DWARF calling convention.

• 3.8: https://github.com/llvm-mirror/llvm/blob/release_38/lib/Bitcode/Reader/BitcodeReader.cpp#L2133
• 6.0: https://github.com/llvm-mirror/llvm/blob/release_60/lib/Bitcode/Reader/MetadataLoader.cpp#L1303

CC

• Record index: 3
• Type: Word8
• Default: 0
• Github commit: llvm-mirror/llvm@3d3aca2
• LLVM review: https://reviews.llvm.org/D21114