Use llvm::APFloat for formatting if a target is available. Each target when...

Use llvm::APFloat for formatting if a target is available. Each target when debugging has a "ASTContext" that helps us to use the correct floating point semantics. Now that APFloat supports toString we now use that. If we don't have a target, we still fall back on the old display methodology, but the important formatting should always have a target available and thus use the compiler floating point code.

Modified the test programs to use floating point constants that always will display correctly. We had some numbers that were being rounded, and now that we are using clang, we no longer round them and we get more correct results.

llvm-svn: 183792