[ADT] Simplify and optimize StringSwitch

This change improves incremental rebuild performance on dual Xeon 8168
machines by 54%. This change also improves run time code gen by not
forcing the case values to be lvalues.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326109 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed