[TypeSwitch/Compiler.h] Provide a LLVM_NODEBUG macro and use it in TypeSwitch.h

TypeSwitch.h is used pervasively in MLIR and often has dozens of types switched
over.  It uses "zero cost" variadic templates to implement the dispatching
mechanism... which isn't zero cost in debug builds, and which causes a massive
problem for actually debugging things that use it - you get dozens of nonsense
frames in the debugger for simple things like a visitor.

Fix this by marking the key method in TypeSwitch as nodebug + alwaysinline.
This resolves LLVM PR49301

Differential Revision: https://reviews.llvm.org/D111520