[clang] Expose CoawaitExpr's operand in the AST

Previously the Expr returned by getOperand() was actually the
subexpression common to the "ready", "suspend", and "resume"
expressions, which often isn't just the operand but e.g.
await_transform() called on the operand.

It's important for the AST to expose the operand as written
in the source for traversals and tools like clangd to work
correctly.

Fixes https://github.com/clangd/clangd/issues/939

Differential Revision: https://reviews.llvm.org/D115187