clear obligations-added flag with nested fulfillcx

[nikomatsakis]

This flag is a debugging measure designed to detect cases where we start
a snapshot, create type variables, register obligations involving those
type variables in the fulfillment cx, and then have to unroll the
snapshot, leaving "dangling type variables" behind. HOWEVER, in some
cases the flag is wrong. In particular, we sometimes create a
"mini-fulfilment-cx" in which we enroll obligations. As long as this
fulfillment cx is fully drained before we return, this is not a problem,
as there won't be any escaping obligations in the main cx. So we add a
fn to save/restore the flag.

Fixes #36053.

r? @arielb1

[arielb1]

error: /build/src/test/compile-fail/issue-36053-2.rs:17: expected error not found: E0281

error: 1 unexpected errors found, 1 expected errors not found
status: exit code: 101
command: /build/build/x86_64-unknown-linux-gnu/stage2/bin/rustc /build/src/test/compile-fail/issue-36053-2.rs -L /build/build/x86_64-unknown-linux-gnu/test/compile-fail --target=x86_64-unknown-linux-gnu --error-format json -L /build/build/x86_64-unknown-linux-gnu/test/compile-fail/issue-36053-2.stage2-x86_64-unknown-linux-gnu.compile-fail.libaux -C prefer-dynamic -o /build/build/x86_64-unknown-linux-gnu/test/compile-fail/issue-36053-2.stage2-x86_64-unknown-linux-gnu -Crpath -O -Lnative=/build/build/x86_64-unknown-linux-gnu/rust-test-helpers
unexpected errors (from JSON output): [
    Error {
        line_num: 17,
        kind: Some(
            Error
        ),
        msg: "17:55: 17:60: no method named `count` found for type `std::iter::Filter<std::iter::Fuse<std::iter::Once<&str>>, [closure@/build/src/test/compile-fail/issue-36053-2.rs:17:39: 17:53]>` in the current scope"
    }
]

not found errors (from test file): [
    Error {
        line_num: 17,
        kind: Some(
            Error
        ),
        msg: "E0281"
    }
]

thread '[compile-fail] compile-fail/issue-36053-2.rs' panicked at 'explicit panic', src/tools/compiletest/src/runtest.rs:1084
note: Run with `RUST_BACKTRACE=1` for a backtrace.


failures:
    [compile-fail] compile-fail/issue-36053-2.rs

test result: FAILED. 2565 passed; 1 failed; 12 ignored; 0 measured

[nikomatsakis]

@arielb1 fixed (I think).

[arielb1]

@bors r+

[bors]

📌 Commit 6353e30 has been approved by arielb1