[LLVM] Add "cl-opt" attribute to target_kind "llvm" (#12440)

* [LLVM] Add "cl-opt" attribute to target_kind "llvm"

Add LLVMTargetInfo class that can be used to query the LLVM
configuration without forcing an LLVMTarget to be created.

There is no programmatic way to obtain the actual type of an LLVM
option. The type is necessary to obtain the value of the option,
hence it must be provided as a part of the option string.
See src/target/llvm/target_kind.cc for more information about the
syntax.

* Fix lowercasing of bool value string

* Use std::optional instead of std::pair<..., bool>

* Treat malformed options as fatal errors

* Fix linter

* More unit tests for option parsing, have one case per test

* Remove "option ignored" from fatal error messages

Author: Krzysztof Parzyszek