[TableGen] Print #nnn as a name of an non-native reg unit with id nnn

[TableGen] Print #nnn as a name of an non-native reg unit with id nnn

When using -debug with -gen-register-info, tablegen will crash when
trying to print a name of a non-native register unit. This patch only
affects the debug information generated while running llvm-tblgen,
and has no impact on the compilable code coming out of it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298875 91177308-0d34-0410-b5e6-96231b3b80d8