stage2: improvements aimed at std lib integration

* AstGen: emit decl lookup ZIR instructions rather than directly
   looking up decls in AstGen. This is necessary because we want to
   reuse the same immutable ZIR code for multiple generic instantiations
   (and comptime function calls).
 * AstGen: fix using members_len instead of fields_len for struct decls.
 * structs: the struct_decl ZIR instruction is now also a block. This is
   so that the type expressions, default field value expressions, and
   alignment expressions can be evaluated in a scope that contains the
   decls from the struct namespace itself.
 * Add "std" and "builtin" packages to the builtin package.
 * Don't try to build glibc, musl, or mingw-w64 when using `-ofmt=c`.
 * builtin.zig is generated without `usingnamespace`.
 * builtin.zig takes advantage of `std.zig.fmtId` for CPU features.
 * A first pass at implementing `usingnamespace`. It's problematic and
   should either be deleted, or polished, before merging this branch.
 * Sema: allow explicitly specifying the namespace in which to look up
   Decls. This is used by `struct_decl` in order to put the decls from
   the struct namespace itself in scope when evaluating the type
   expressions, default value expressions, and alignment expressions.
 * Module: fix `analyzeNamespace` assuming that it is the top-level root
   declaration node.
 * Sema: implement comptime and runtime cmp operator.
 * Sema: implement peer type resolution for enums and enum literals.
 * Pull in the changes from master branch:
   262e09c482.
 * ZIR: complete out simple_ptr_type debug printing

src/codegen/spirv/spec.zig

@@ -21,7 +21,7 @@
 // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 // FROM,OUT OF OR IN CONNECTION WITH THE MATERIALS OR THE USE OR OTHER DEALINGS
 // IN THE MATERIALS.
-const Version = @import("builtin").Version;
+const Version = @import("std").builtin.Version;
 pub const version = Version{ .major = 1, .minor = 5, .patch = 4 };
 pub const magic_number: u32 = 0x07230203;
 pub const Opcode = extern enum(u16) {