Map literal syntax sugar

[hanbings-agent]

feat: Introduce feature-gated map literal syntax sugar

This PR introduces a new feature-gated syntax sugar for initializing map-like collections, addressing long-standing requests (e.g., RFCs rust-lang#1420, rust-lang#542, rust-lang#1602).

Why this change?
To simplify the initialization of HashMap, BTreeMap, and other types implementing FromIterator, providing a more concise syntax akin to vec![].

What does it do?
It allows initializing collections using { k1, v1, k2, v2, ... } syntax, which desugars in the parser to ::core::iter::FromIterator::from_iter([(k1, v1), (k2, v2), ...]).

Key implementation details:

• Parser Desugaring: The syntax is directly desugared into a FromIterator::from_iter call on an array of tuples, preserving evaluation order and leveraging existing FromIterator implementations. This avoids changes to AST/HIR.
• Conflict Resolution: A snapshot-based probing mechanism is used in the parser to differentiate this new syntax from existing block expressions (e.g., { let ... }, { use ... }, { #[attr] ... }), preventing parsing conflicts and spurious diagnostics.
• Feature Gate: The syntax is controlled by #![feature(map_literals)].

Bootstrap Environment Fixes:
Includes minor adjustments to src/bootstrap to ensure x build and x check pass reliably in environments with non-standard C/C++ toolchain configurations, specifically by allowing x check to inherit CC/CXX environment variables and providing fallbacks for libcxx-version compilation.

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[cursor]

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