Fixes how (very) large size entries in the TYPE_CODE_NUMENTRY is
handled when reading bitcode. Makes sure that we con't call
vector.resize() with too large a value (replacing an allocation
exception with a parse error).

Also tries to clean up type modeling of bitcode indices (references to
values etc in the bitcode). Uses common type NaClBcIndexSize_t and
NaClRelBcIndexSize_t (defined in nacl) to describe these (32-bit)
values.

Note: We use cast truncation of 64-bit values to NaClBcIndexSize_t and
NaClRelBcIndexSize_t, since negative value indices are stored both as
32 and 64 bit values. The truncation cast handles this differences
correctly (and efficiently).

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4195
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1182323011