Fixes the PNaCl bitcode reader to maintain two lists of global
variables. The first, VariableDeclarations, is the list of
variable declarations to be lowered by the emitter. The second,
ValueIDConstants, is the corresponding constant symbol to use
when references to the corresponding global variable declaration
is referenced when processing functions.

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

Review URL: https://codereview.chromium.org/883673005