Add latest druntime change
Add IF_LOG to more logging statements.

Evaluating arguments which call toChars() or toPrettyChars() are much more expensive then checking Logger::enabled().
Enable inlining of Logger::enabled().

This should help with performance of previous commit.
Implement longdouble as a wrapper around llvm::APFloat.

This commit introduces a new class ldc::longdouble which provides real_t
semantics based on the target. It uses llvm::APFloat as base implementation.

This fixes issue #259.