Add a PLACEHOLDER code kind.

Add a PLACEHOLDER code kind.

The PLACEHOLDER code kind is used when compiling a code object that has
direct calls to other code objects, but those other code objects do not
yet exist because they have not yet been compiled. It serves as a
placeholder to break the cycle, e.g. in WASM.

R=yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1308393003

Cr-Commit-Position: refs/heads/master@{#30348}