TableGen/SearchableTables: Cast enums to unsigned in generated code

Summary:
This should fix signedness warnings when compiling with MSVC.

Change-Id: I4664cce0ba91e9b42d21a86fd4a7e82f2320c451

Reviewers: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51097

llvm-svn: 340518