[clang] [llvm] [Clang][OpenMP] Support for dispatch construct (Sema & Codegen) support (PR #131838)

[Deepak Eachempati via llvm-commits]

dreachem wrote:

@alexey-bataev For something like this:

```c++
void f()
{
    #pragma omp dispatch nowait depend(inout:x)
    g();
}
```

The AST dump for that function is as follows, with @SunilKuravinakop's patch:

```
`-FunctionDecl 0xed2dac8 <line:7:1, line:11:1> line:7:6 f 'void ()'
  `-CompoundStmt 0xed2df28 <line:8:1, line:11:1>
    `-CompoundStmt 0xed2df08 <<invalid sloc>>
      |-OMPTaskwaitDirective 0xed2ded0 <line:9:5, col:48> openmp_standalone_directive
      | `-OMPDependClause 0xed2dbe8 <col:33, col:47>
      |   `-DeclRefExpr 0xed2db80 <col:46> 'int' lvalue Var 0xed2da38 'x' 'int'
      `-CapturedStmt 0xed2dea8 <line:10:5, col:7>
        `-CapturedDecl 0xed2dd18 <<invalid sloc>> <invalid sloc>
          |-CallExpr 0xed2de88 <col:5, col:7> 'void'
          | `-ImplicitCastExpr 0xed2de70 <col:5> 'void (*)()' <FunctionToPointerDecay>
          |   `-DeclRefExpr 0xed2de20 <col:5> 'void ()' Function 0xed2d7e0 'g' 'void ()'
          `-ImplicitParamDecl 0xed2ddb0 <line:9:5> col:5 implicit __context 'struct (unnamed at dispatch.c:9:5) *const restrict'
```

Should the AST instead look something like this?

```
`-FunctionDecl 0xf804ac8 <line:7:1, line:11:1> line:7:6 f 'void ()'
  `-CompoundStmt 0xf804ea8 <line:8:1, line:11:1>
    `-OMPDispatchDirective 0xf804e60 <line:9:5, col:57>
      |-OMPNowaitClause 0xf804b70 <col:26, col:57>'
      |-OMPDependClause ...
      `-CapturedStmt 0xf804e08 <line:10:5, col:7>
        `-CapturedDecl 0xf804c78 <<invalid sloc>> <invalid sloc>
          |-CallExpr 0xf804de8 <col:5, col:7> 'void'
          | `-ImplicitCastExpr 0xf804dd0 <col:5> 'void (*)()' <FunctionToPointerDecay>
          |   `-DeclRefExpr 0xf804d80 <col:5> 'void ()' Function 0xf8047e0 'g' 'void ()'
          `-ImplicitParamDecl 0xf804d10 <line:9:5> col:5 implicit __context 'struct (unnamed at dispatch.c:9:5) *const restrict'
```

and then handle the implicit taskwait in codegen? I am also confused how an additional CapturedStmt to represent the taskwait should be implemented in Sema. 

https://github.com/llvm/llvm-project/pull/131838