[GitHub] Add Copilot review instructions for LLDB (#165783)

This is an experiment to encode the LLVM Coding Standards [1] as
instructions for the Copilot reviewer on GitHub. Ideally, this will
catch common issues automatically and reduce the review burden.

Adding Copilot as a reviewer is entirely opt-in. Initially, I will add
it as a reviewer to test this. If the experiment is successful, we can
explore how to integrate this into other parts of LLVM.

[1]: https://llvm.org/docs/CodingStandards.html

.github/instructions/lldb.instructions.md

@@ -0,0 +1,79 @@
+---
+applyTo: lldb/**/*
+---
+
+When reviewing code, focus on:
+
+## Language, Libraries & Standards
+
+- Target C++17 and avoid vendor-specific extensions.
+- For Python scripts, follow PEP 8.
+- Prefer standard library or LLVM support libraries instead of reinventing data structures.
+
+## Comments & Documentation
+
+- Each source file should include the standard LLVM file header.
+- Header files must have proper header guards.
+- Non-trivial classes and public methods should have Doxygen documentation.
+- Use `//` or `///` comments normally; avoid block comments unless necessary.
+- Non-trivial code should have comments explaining what it does and why. Avoid comments that explain how it does it at a micro level.
+
+## Language & Compiler Issues
+
+- Write portable code; wrap non-portable code in interfaces.
+- Do not use RTTI or exceptions.
+- Prefer C++-style casts over C-style casts.
+- Do not use static constructors.
+- Use `class` or `struct` consistently; `struct` only for all-public data.
+- When then same class is declared or defined multiple times, make sure it's consistently done using either `class` or `struct`.
+
+## Headers & Library Layering
+
+- Include order: module header → local/private headers → project headers → system headers.
+- Headers must compile standalone (include all dependencies).
+- Maintain proper library layering; avoid circular dependencies.
+- Include minimally; use forward declarations where possible.
+- Keep internal headers private to modules.
+- Use full namespace qualifiers for out-of-line definitions.
+
+## Control Flow & Structure
+
+- Prefer early exits over deep nesting.
+- Do not use `else` after `return`, `continue`, `break`, or `goto`.
+- Encapsulate loops that compute predicates into helper functions.
+
+## Naming
+
+- LLDB's code style differs from LLVM's coding style.
+- Variables are `snake_case`.
+- Functions and methods are `UpperCamelCase`.
+- Static, global and member variables have `s_`, `g_` and `m_` prefixes respectively.
+
+## General Guidelines
+
+- Use `assert` liberally; prefer `llvm_unreachable` for unreachable states.
+- Do not use `using namespace std;` in headers.
+- Provide a virtual method anchor for classes defined in headers.
+- Do not use default labels in fully covered switches over enumerations.
+- Use range-based for loops wherever possible.
+- Capture `end()` outside loops if not using range-based iteration.
+- Including `<iostream>` is forbidded. Use LLVM’s `raw_ostream` instead.
+- Don’t use `inline` when defining a function in a class definition.
+
+## Microscopic Details
+
+- Preserve existing style in modified code.
+- Prefer pre-increment (`++i`) when value is unused.
+- Use `private`, `protected`, or `public` keyword as appropriate to restrict class member visibility.
+- Omit braces for single-statement `if`, `else`, `while`, `for` unless needed.
+
+## Review Style
+
+- Be specific and actionable in feedback.
+- Explain the "why" behind recommendations.
+- Link back to the LLVM Coding Standards: https://llvm.org/docs/CodingStandards.html.
+- Ask clarifying questions when code intent is unclear.
+
+Ignore formatting and assume that's handled by external tools like `clang-format` and `black`.
+Remember that these standards are **guidelines**.
+Always prioritize consistency with the style that is already being used by the surrounding code.