DMD source violates c++11 standards.

[Hans W. Uhlig]

This also makes compiling LDC with clang rather difficult

[  1%] Building CXX object CMakeFiles/LDCShared.dir/dmd2/func.c.o
/root/llvm/src/ldc/dmd2/func.c:540:18: error: case value 
evaluates to -2, which cannot be narrowed to type 'size_t' (aka 
'unsigned long') [-Wc++11-narrowing]
             case -2:    // can't determine because of fwd refs
                  ^
/root/llvm/src/ldc/dmd2/func.c:503:18: error: case value 
evaluates to -1, which cannot be narrowed to type 'size_t' (aka 
'unsigned long') [-Wc++11-narrowing]
             case -1:
                  ^
/root/llvm/src/ldc/dmd2/func.c:632:22: error: case value 
evaluates to -2, which cannot be narrowed to type 'size_t' (aka 
'unsigned long') [-Wc++11-narrowing]
                 case -2:
                      ^
/root/llvm/src/ldc/dmd2/func.c:629:22: error: case value 
evaluates to -1, which cannot be narrowed to type 'size_t' (aka 
'unsigned long') [-Wc++11-narrowing]
                 case -1: