[llvm] [BOLT] Add option --merge-text-sections (PR #217732)

[YongKang Zhu via llvm-commits]

yozhu wrote:

> > How does this option interact with `-use-old-text`?
> 
> It is independent of whether or not the old text section will be reused. It will merge new text sections that BOLT generates, like `.text` and `.text.old`, which can be written to the original text section due to `--use-old-text`.

Will add a case in the test to cover `--use-old-text`.

https://github.com/llvm/llvm-project/pull/217732