Help LLM track IR inst creation during debugging

[csyonghe]

We have the _slangIRAllocBreak mechanism to help developers break the compiler when a specific instruction is being created, so we can inspect the callstack and understand why the problematic instruction is being created.

This is a useful debugging tool, but it is not as easy for LLMs to use it.

We can introduce an environment variable, SLANG_DEBUG_IR_BREAK. During creation of Slang global session, we can read this environment variable and assign _slangIRAllocBreak to the value stored in the environment variable. We can then introduce another global variable, bool _slangIRPrintStackAtBreak = false;, and set it to true when the environment variable exists.

Then in uint32_t _debugGetAndIncreaseInstCounter() after line 1757, call backtrace() when on linux platform (#ifdef SLANG_LINUX_FAMILY). This will allow us to run slangc or slang-test with SLANG_DEBUG_IR_BREAK=xxx environment variable and then see the stack dump when an instruction with the specified UID is being created.