[CIR][CIRGen] Use Clang Codegen's skeleton in CIRGenFunction::buildBu…

…iltinExpr (llvm#967)

This PR helps us to triage unimplemented builtins (that are target
independent).
There are unhandled builtins in CIR Codegen
`[CIRGenFunction::buildBuiltinExpr](https://github.com/llvm/clangir/blob/4c446b3287895879da598e23164d338d04bced3e/clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp#L305)`.
And those builtins have implementation in
[OG](https://github.com/llvm/clangir/blob/4c446b3287895879da598e23164d338d04bced3e/clang/lib/CodeGen/CGBuiltin.cpp#L2573).
Currently, those builtins just are treated as LibraryCall or some other
ways which eventually get failure, and failure messages are confusing.
This PR address this problem by refactoring
`CIRGenFunction::buildBuiltinExpr` to keep the same skeleton as OG
counterpart `CodeGenFunction::EmitBuiltinExpr`, and add builtin name to
NYI message