Add LLVM support for reading and writing DFP constant values in LLVM IR text

[tahonermann]

Changes made in PR #33 add support for DFP representation in the LLVM IR. This issue tracks adding support for reading and writing constant values of DFP type in LLVM IR text.

[tahonermann]

In addition to other updates, this will require updates to:

• llvm::LLLexer::LexDigitOrNegative()
• llvm::LLLexer::LexPositive()