Fix build compatibility with macOS 26 and Apple Clang 21

[ikkoham]

Summary

macOS 26 (Tahoe) SDK removed __has_builtin guards from libc++ headers (e.g., __builtin_ctzg, __is_nothrow_convertible), making them incompatible with LLVM 16's clang. Additionally, Apple Clang 21 introduced new warnings that break the build under -Werror.

This PR fixes both issues so that CUDA-Q builds and passes all tests on macOS 26 with Apple Clang 21.

LLVM libc++ runtimes (SDK 26+)

• set_env_defaults.sh: Auto-detect active SDK version via xcrun --show-sdk-version. When SDK >= 26, include runtimes in LLVM_PROJECTS to build LLVM's own libc++.
• cudaq-quake.cpp: Use -nostdinc++ to suppress SDK C++ headers when LLVM's libc++ is available, while keeping -isysroot for C standard headers.
• nvq++.in: Add -Wl,-syslibroot for ld64.lld to find libSystem in the SDK. Add -lc++abi during final link when LLVM's libc++abi is present.
• build_llvm.sh: Do not bake SDK sysroot paths into clang++.cfg on macOS (they become stale after Xcode updates). Sysroot is resolved at runtime by nvq++.

Relocatable linking

• nvq++.in: LLVM 16's ld64.lld does not implement -r (relocatable linking). Probe the configured linker and fall back to system ld for the object merge step if needed.
• device_call.cpp: Update FileCheck pattern to match both Apple ld and ld64.lld error formats.

Apple Clang 21 warnings

• CMakeLists.txt: Add -Wno-character-conversion for gtest (third-party).
• server_impl/CMakeLists.txt: Add -Wno-deprecated-literal-operator for Crow (third-party).
• cudaq.cpp: Fix -Wnontrivial-memcall on memset with std::vector<bool>.
• vqe_tester.cpp: Add missing #pragma to suppress -Wdeprecated-declarations for backward-compatibility test (matching existing pattern in builder_tester.cpp).

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[sacpis]

/ok to test d9566e7

Command Bot: Processing...

[github-actions]

CUDA Quantum Docs Bot: A preview of the documentation can be found here.

[sacpis]

/ok to test b53ad52

Command Bot: Processing...

[github-actions]

CUDA Quantum Docs Bot: A preview of the documentation can be found here.

[taalexander]

Thanks for putting this together Ikko. I don't have my macOS machine with me this week but will test and review next week.