[llvm] [CGData] Global Merge Functions (PR #112671)

[Kyungwoo Lee via llvm-commits]

kyulee-com wrote:

> Could please add one or more rst files to https://github.com/llvm/llvm-project/tree/main/llvm/docs about cgdata. Tools, passes, flags, and more. For better visibility.

We already have a few docs available:
 - Clang flags about `-fcodegen-data-generate`and `-fcodegen-data-use`: https://github.com/llvm/llvm-project/blob/main/clang/docs/UsersManual.rst
 - `llvm-cgdata` tool: https://github.com/llvm/llvm-project/blob/main/llvm/docs/CommandGuide/llvm-cgdata.rst
 
However, these are somewhat fragmented. Once all flags and linker support are finalized, I plan to create a separate rst document for cgdata, including more examples and usage guidelines.

https://github.com/llvm/llvm-project/pull/112671