Fix quadratic runtime when adding items to tooling::Replacements.

Fix quadratic runtime when adding items to tooling::Replacements.

Previously, we would search through all replacements when inserting a
new one to check for overlaps. Instead, make use of the fact that we
already have a set of replacments without overlaps to find the potential
overlap with lower_bound.

Differential Revision: https://reviews.llvm.org/D23119

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@277597 91177308-0d34-0410-b5e6-96231b3b80d8