Add generated_link_name_override method for ParseCallbacks.

[ghost]

generated_link_name_override lets the developer choose the link name for
a symbol.

If a link name is specified, the attribute #[link_name = "\u{1}VALUE"] will
be set, overriding any other potential link name, including the mangled name.

This commit also adds an option to the bindgen cli called prefix-link-name,
to prefix the link name of exported symbols.

I think this should properly solve #1375.

[pvdrz]

tests aren't passing. Do you have any idea why?

[bors-servo]

☔ The latest upstream changes (presumably a8c8638) made this pull request unmergeable. Please resolve the merge conflicts.

[ghost]

tests aren't passing. Do you have any idea why?

it seems that bindgen doesn't output the same rust code for the cpp tests for every host platforms. I'm investigating.

[ghost]

Got some update: it was failing because of the roundtrip test.
Then I undertood the purpose of the parse_callbacks mod, so I added a parse callback called PrefixLinkNameParseCallback to solve the issue.

[bors-servo]

☔ The latest upstream changes (presumably f019a9b) made this pull request unmergeable. Please resolve the merge conflicts.

[bors-servo]

☔ The latest upstream changes (presumably a101063) made this pull request unmergeable. Please resolve the merge conflicts.

[pvdrz]

👋 hey I checked your PR and I was wondering if the --prefix-link-name flag is worth it considering that it will be used to just remove the \u{1} prefix. Do you actually need to prefix something to the link name of every function?

[ghost]

👋 hey I checked your PR and I was wondering if the --prefix-link-name flag is worth it considering that it will be used to just remove the \u{1} prefix. Do you actually need to prefix something to the link name of every function?

hey!
In fact it doesn't remove the \u{1} prefix, or did I misunderstand what you meant?

The idea behind prefixing the link name of every function and variables is to be able to link against a library that has been modified by objcopy.
For instance it could be use to avoid C symbol name collisions when you link a rust library that "embeds" a C library against another library that also embeds the very same library.