[clang][ExtractAPI] Modify declaration fragment methods to add a new fragment at an arbitrary offset.

The current implementation doesn't support merging declaration fragments at arbitrary offsets. This patch adds that support
by modifying declaration fragment methods.

Differential Revision: https://reviews.llvm.org/D151048

GitOrigin-RevId: 135ce2f820d881d5a7c5d90feab109174918a21f
2 files changed