Disallow overlapping inserts, even when inserting the same value.

We always disallowed overlapping inserts with different values, and this makes
the insertion code smaller and faster.

If an overwriting insert is needed, it can be added as a separate method that
trims any existing intervals before inserting. The immediate use cases for
IntervalMap don't need this - they only use disjoint insertions.

llvm-svn: 120264