Supports adding insertion around non-insertion replacements.

Summary:
Extend `tooling::Replacements::add()` to support adding order-independent replacements.

Two replacements are considered order-independent if one of the following conditions is true:
  - They do not overlap. (This is already supported.)
  - One replacement is insertion, and the other is a replacement with
    length > 0, and the insertion is adjecent to but not contained in the
    other replacement. In this case, the replacement should always change
    the original code instead of the inserted text.

Reviewers: klimek, djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D24515

llvm-svn: 281457