Fix compiler warnings.

Building spir-tools failed on Debian wheezy with: 
```
 [ 90%] Building CXX object tools/spir-tools/spir_name_mangler/CMakeFiles/SpirNameMangler.dir/FunctionDescriptor.cpp.o
In file included from /home/kraiskil/llvm/spir/llvm-3.2.src/tools/spir-tools/spir_name_mangler/FunctionDescriptor.h:11:0,
                 from /home/kraiskil/llvm/spir/llvm-3.2.src/tools/spir-tools/spir_name_mangler/FunctionDescriptor.cpp:8:
/home/kraiskil/llvm/spir/llvm-3.2.src/tools/spir-tools/spir_name_mangler/ParameterType.h:202:47: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
/home/kraiskil/llvm/spir/llvm-3.2.src/tools/spir-tools/spir_name_mangler/ParameterType.h:208:10: error: extra qualification ‘SPIR::PointerType::’ on member ‘setQualifier’ [-fpermissive]
make[2]: *** [tools/spir-tools/spir_name_mangler/CMakeFiles/SpirNameMangler.dir/FunctionDescriptor.cpp.o] Fel 1
make[1]: *** [tools/spir-tools/spir_name_mangler/CMakeFiles/SpirNameMangler.dir/all] Fel 2
make: *** [all] Fel 2
```
After the attached patches, it compiles fine.