[clang] [Clang][RISCV] Mangle the ABI tag of a standard calling convention variant (PR #217249)

[Kito Cheng via cfe-commits]

kito-cheng wrote:

Changes:
- Rebase
- Add testcase to make sure multiple ABI tag work with riscv_vls_cc correctly 
```
// CHECK-DAG: @_Z7tagged2B3abcB16riscv_vls_cc_128B4userv
__attribute__((abi_tag("user", "abc"), riscv_vls_cc(128))) void tagged2() {}
```

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Will merge once CI happy.

https://github.com/llvm/llvm-project/pull/217249