TableGen: Let getAllDerivedDefinitions() numeric order.

Since `RK::Recs` is sorted by character order, anonymous defs will be
enumerated like this;

  - anonymous_0
  - anonymous_1
  - anonymous_10
  - anonymous_100
  - anonymous_1000
  - ...
  - anonymous_99
  - anonymous_990
  - ...
  - anonymous_999

Some records around each gap might be wrapped around along increase or
decrease of records in middle. Then output order of anonymous defs
might be changed.

Numeric sort is expected to prevent such wrap-arounds.
This can be implemented with `StringRef::compare_numeric()`.

  - ...
  - anonymous_99
  - anonymous_100
  - ...
  - anonymous_999
  - anonymous_1000
  - ...

See also discussions in D145874.

Differential Revision: https://reviews.llvm.org/D145874