[clang-tools-extra] [clang-tools-extra][ExtractAPI] create clang-symbolgraph-merger (PR #65894)

[via cfe-commits]

Arsenic-ATG wrote:

- The changes were initially being tracked on llvm Phabricator 
Differential Revision: https://reviews.llvm.org/D158646
- Dependent on #65891 
- Currently keeping this as draft pull request and would be opening it for review once it is ready and tested completely.

https://github.com/llvm/llvm-project/pull/65894