merge ambiguity errors that blame the same inference variable

[Albab-Hasan]

multiple FulfillmentErrorCode::Ambiguity obligations on the same inference variable only ever reported the first. the rest got dropped after set_tainted_by_errors fired. grouped them by sub-unification-table root and fold Trait/Projection predicates from the group into notes on the primary. uses the sub root not the raw TyVid so variables unified by a pending Coerce obligation correctly land in the same group.

fixes #103911

r? @lcnr

[lcnr]

looks vaguely good to me, gonna reassign though as I don't have the capacity to properly review this rn

@rustbot reroll

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

...............................F.................. (50/144)
.................................................. (100/144)
............................................       (144/144)

======== tests/rustdoc-gui/go-to-collapsed-elem.goml ========

[ERROR] line 40
    at `tests/rustdoc-gui/go-to-collapsed-elem.goml` line 21: Error: Node is either not clickable or not an Element: for command `click: "//*[@id='search']//a[@href='../test_docs/struct.Foo.html#method.must_use']"`
    at <file:///checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/test_docs/struct.Foo.html?search=t_use>


<= doc-ui tests done: 143 succeeded, 1 failed, 0 filtered out

Error: ()

[Albab-Hasan]

@jackh726 i think this is causing the failure:

#157747

[Albab-Hasan]

@jackh726 hello, just wanted to follow up on the pr. sorry if this is bothering you