[clang] [CIR] Upstream initial function call support (PR #134673)

[Bruno Cardoso Lopes via cfe-commits]

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+// RUN: cir-opt %s | FileCheck %s
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bcardosolopes wrote:

Not a review item, more like a general "it will be nice comment": `mlir-opt` has a `--verify-roundtrip` flag, which we should add at some point to `cir-opt` to make such testings even more robust.

https://github.com/llvm/llvm-project/pull/134673