[clang] [llvm] [RISCV] Implement the implications of C extension (PR #132259)

[Kito Cheng via llvm-commits]

kito-cheng wrote:

(Disclaimer: I am not intend to ignite the flames of war about the ISA string canonicalization!)

Give few more background behind the PR, the issue we want to resolve is the multilib issue: we starting using `zc*` extension, but we also have user for using `c` extension, however we found either `rv64ic` or `rv64izca` can't cover each other, but they are identical in theory.

In my mind there is a best way to fix that: let c implied `zca`, ***AND*** also add `c` back when the condition met`*`, however the later one is more disturb I guess, so do the first part, which is what this PR do, and then always use `zc*` extension when building multllib list, that could at least resolve the multilib issue between `zc*` and `c`.

`*` The condition in the [spec](https://github.com/riscvarchive/riscv-code-size-reduction/blob/main/Zc-specification/Zc.adoc#13-c):
The C extension is the superset of the following extensions:
- Zca
- Zcf if F is specified (RV32 only)
- Zcd if D is specified

https://github.com/llvm/llvm-project/pull/132259