[clang] [OpenMP][CIR] Implement basic 'parallel' lowering + some clause infra (PR #172308)

[Erich Keane via cfe-commits]

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+// RUN: not %clang_cc1 -fopenmp -emit-cir -fclangir %s -o - | FileCheck %s
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erichkeane wrote:

It doesn't 'crash' the compiler, it causes `not yet implemented` errors to be emitted, which stops translation.  We need the 'not' so that FileCheck will check what we've emitted anyway.

https://github.com/llvm/llvm-project/pull/172308