Fix missing direct includes

[Aesthete]

Problem

Embedding Luau in a project that disables libc++ transitive includes fails to compile because several Luau files use declarations without including the headers that define them.

Reproduced on Linux with Clang 21.1.8, libc++ 21.1.8, C++17, and:

CMAKE_CXX_FLAGS="-stdlib=libc++ -D_LIBCPP_REMOVE_TRANSITIVE_INCLUDES"
CMAKE_EXE_LINKER_FLAGS="-stdlib=libc++"

Example errors include undeclared abs, malloc, atoi, strtoull, errno, getenv, free, and exit.

ConditionA64.h uses CODEGEN_ASSERT, defined in CodeGenCommon.h. It's unrelated to the libc includes but it's also not IWYU and appeared during testing.

Fix

Add the required direct includes.

Tests

Luau.UnitTest, Luau.Conformance, and Luau.CLI.Test pass with the same flags.

[Un1q32]

In libc++ 23, removing transitive includes became the default, and in libc++ 24 it will be mandatory.