Update Symtab::InitAddressIndexes so that computed symbol sizes

Update Symtab::InitAddressIndexes so that computed symbol sizes
will not exceed the bounds of their Section.  This is addressing a
problem where a file had a large space between two sections that
were not used by this module - the last symbol in the text section
had an enormous size because the distance between that and the first
symbol in the data section were used to compute the size.

http://reviews.llvm.org/D19004
<rdar://problem/25227945>

llvm-svn: 266165