link: support exporting constant values without a Decl

[andrewrk]

The main motivating change here is to prevent the creation of a fake Decl object by the frontend in order to @export() a value.

Instead, link.updateDeclExports is renamed to link.updateExports and accepts a tagged union which can be either a Decl.Index or a InternPool.Index.

With this, the only remaining calls to startAnonDecl() are for comptime value mutation:

$ grep -RI startAnonDecl ../src
../src/Sema.zig:    pub fn startAnonDecl(block: *Block) !WipAnonDecl {
../src/Sema.zig:    var anon_decl = try block.startAnonDecl(); // TODO: comptime value mutation without Decl
../src/Sema.zig:        var anon_decl = try block.startAnonDecl(); // TODO: comptime value mutation without Decl
../src/Sema.zig:    var anon_decl = try block.startAnonDecl(); // TODO: comptime value mutation without Decl