stage2: rename ir.zig to air.zig

We've settled on the nomenclature for the artifacts the compiler
pipeline produces:

1. Tokens
2. AST (Abstract Syntax Tree)
3. ZIR (Zig Intermediate Representation)
4. AIR (Analyzed Intermediate Representation)
5. Machine Code

Renaming `ir` identifiers to `air` will come with the inevitable
air-memory-layout branch that I plan to start after the 0.8.0 release.

src/codegen/c.zig

@@ -6,7 +6,7 @@ const log = std.log.scoped(.c);
 const link = @import("../link.zig");
 const Module = @import("../Module.zig");
 const Compilation = @import("../Compilation.zig");
-const ir = @import("../ir.zig");
+const ir = @import("../air.zig");
 const Inst = ir.Inst;
 const Value = @import("../value.zig").Value;
 const Type = @import("../type.zig").Type;

src/codegen/llvm.zig

@@ -9,7 +9,7 @@ const math = std.math;
 
 const Module = @import("../Module.zig");
 const TypedValue = @import("../TypedValue.zig");
-const ir = @import("../ir.zig");
+const ir = @import("../air.zig");
 const Inst = ir.Inst;
 
 const Value = @import("../value.zig").Value;

src/codegen/spirv.zig

@@ -11,7 +11,7 @@ const Decl = Module.Decl;
 const Type = @import("../type.zig").Type;
 const Value = @import("../value.zig").Value;
 const LazySrcLoc = Module.LazySrcLoc;
-const ir = @import("../ir.zig");
+const ir = @import("../air.zig");
 const Inst = ir.Inst;
 
 pub const TypeMap = std.HashMap(Type, u32, Type.hash, Type.eql, std.hash_map.default_max_load_percentage);

src/codegen/wasm.zig

@@ -9,7 +9,7 @@ const wasm = std.wasm;
 
 const Module = @import("../Module.zig");
 const Decl = Module.Decl;
-const ir = @import("../ir.zig");
+const ir = @import("../air.zig");
 const Inst = ir.Inst;
 const Type = @import("../type.zig").Type;
 const Value = @import("../value.zig").Value;