[clang] Add per-global code model attribute (#72078)

This patch adds a per-global code model attribute, which can override
the target's code model to access global variables.

Currently, the code model attribute is only supported on LoongArch. This
patch also maps GCC's code model names to LLVM's, which allows for
better compatibility between the two compilers.


Suggested-by: Arthur Eubanks <aeubanks@google.com>
Link:
https://discourse.llvm.org/t/how-to-best-implement-code-model-overriding-for-certain-values/71816
Link:
https://discourse.llvm.org/t/rfc-add-per-global-code-model-attribute/74944



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Signed-off-by: WANG Rui <wangrui@loongson.cn>