[clang] [CIR] Build out AST consumer patterns to reach the entry point into CIRGen (PR #91007)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> What part of the "camelBack" is visible to the user of the API?  (we're not talking about method names or class names here I believe)

Parameter names, which show up in tooltips in IDEs, etc.

But again, the important thing is to at least be self-consistent. e.g.,
```
  CIRGenModule(mlir::MLIRContext &context, clang::ASTContext &astctx,
               const clang::CodeGenOptions &CGO,
               clang::DiagnosticsEngine &Diags);
```
should be:
```
  CIRGenModule(mlir::MLIRContext &Context, clang::ASTContext &AstCtx,
               const clang::CodeGenOptions &CGO,
               clang::DiagnosticsEngine &Diags);
```
or
```
  CIRGenModule(mlir::MLIRContext &context, clang::ASTContext &astctx,
               const clang::CodeGenOptions &cgo
               clang::DiagnosticsEngine &diags);
```
rather than in a mixed form as it currently is.

https://github.com/llvm/llvm-project/pull/91007