[DWARF] Simplify LineTable::findRowInSeq

We want the last row whose address is less than or equal to Address.
This can be computed as upper_bound - 1, which is simpler than
lower_bound followed by skipping equal rows in a loop.

Since FirstRow (LowPC) does not satisfy the predicate (OrderByAddress)
while LastRow-1 (HighPC) satisfies the predicate. We can decrease the
search range by two, i.e.

upper_bound [FirstRow,LastRow) = upper_bound [FirstRow+1,LastRow-1)

llvm-svn: 358053