Cleans up code by removing unnecessary fields/data structures in top-level parser of Subzero. In particular:

1) Uses FunctionDeclarationList.size() instead of NumFunctionIds.

2) Removes the need for vector DefiningFunctionDeclarationList. Instead uses an (incremented) index NextDefiningFunctionID into vector
FunctionDeclarationList.

BUG=None
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/883493002