`link.Dwarf` loads AST when emitting debug info

[mlugg]

The following calls to Zcu.File.getTree must be eliminated:

zig/src/link/Dwarf.zig

Line 2118 in 1a178d4

const tree = try file.getTree(dwarf.gpa);

zig/src/link/Dwarf.zig

Line 2168 in 1a178d4

const tree = try file.getTree(dwarf.gpa);

zig/src/link/Dwarf.zig

Line 2216 in 1a178d4

const tree = try file.getTree(dwarf.gpa);

zig/src/link/Dwarf.zig

Line 2480 in 1a178d4

const tree = try file.getTree(dwarf.gpa);

This will require adding column information to ZIR declaration instructions, probably through a src_column: u32 field here:

zig/lib/std/zig/Zir.zig

Lines 2581 to 2590 in 1a178d4

	pub const Declaration = struct {
	// These fields should be concatenated and reinterpreted as a `std.zig.SrcHash`.
	src_hash_0: u32,
	src_hash_1: u32,
	src_hash_2: u32,
	src_hash_3: u32,
	/// The name of this `Decl`. Also indicates whether it is a test, comptime block, etc.
	name: Name,
	src_line: u32,
	flags: Flags,

Labeling as a bug because the compiler should never call getTree on a ZIR cache hit without compile errors in the file in question.