[cleanup] Remove LDC-only #if 0 block.
[cleanup] Adjust some whitespace to match DMD.
[cleanup] Remove some unused LDC-only forward decls.
[cleanup] Remove unused LDC-only Expression::isAssignExp.
Remove LDC-only VarDeclaration::semantic3.

This was previously needed for availableExternally, but should
not be relevant any longer.
Merge pull request #734 from klickverbot/cleanup

DMD diff reduction
There is no real need in FuncDeclaration::labmap
Remove VarDeclaration::aggrIndex
Move debugVariable and debugFunc from VarDeclaration to IrVar
FuncDeclaration::inlineAsm is not used
Also move nakedUse from VarDeclaration to IrGlobal
Make FuncDeclaration::isIntrinsic and FuncDeclaration::isVaIntrinsic free functions
Move irFty fields to backend ir classes
Remove old unused declarations