Fix OpenCL/SPIR kernel_arg_type metadata node

This fixes a bug where kernel_arg_type was always changing 'unsigned ' to 'u'
for any parameter type, including non-canonical types.

Example:
  typedef unsigned int myunsignedint;
would report:
  "myunt"


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214305 91177308-0d34-0410-b5e6-96231b3b80d8