[clang] [flang] [llvm] [mlir] [Support][NFC] Move OptimizationLevel to the Support directory (PR #157057)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

> I always thought of ADT and Support libraries as being useful for other projects

That would be nice, and I'd love to move libSupport at the top-level (only the pieces that matches the "generic aspect" of it), but the reality today is that beyond filesystem abstractions, thread pool, and other generic things, there are also things like `X86DisassemblerDecoderCommon.h`, `MSP430Attributes.h`, `AMDHSAKernelDescriptor.h`, `AArch64AttributeParser.h`, `RISCVISAUtils.h`, etc.

https://github.com/llvm/llvm-project/pull/157057