prevent redundant AstGen compile errors

[andrewrk]

Example:

--- a/lib/std/crypto/hkdf.zig
+++ b/lib/std/crypto/hkdf.zig
@@ -10,7 +10,7 @@ const hmac = std.crypto.auth.hmac;
 const mem = std.mem;
 
 /// HKDF-SHA256
-pub const HkdfSha256 = Hkdf(hmac.sha2.HmacSha256);
+pub const HkdfSha256 = comptime Hkdf(hmac.sha2.HmacSha256);
 
 /// HKDF-SHA512
 pub const HkdfSha512 = Hkdf(hmac.sha2.HmacSha512);

[nix-shell:~/dev/zig/build]$ ./zig build-exe test.zig
error: sub-compilation of compiler_rt failed
    crypto/hkdf.zig:13:24: error: redundant comptime keyword in already comptime scope
pub const HkdfSha256 = comptime Hkdf(hmac.sha2.HmacSha256);
                       ^
error: sub-compilation of zig_libc failed
    crypto/hkdf.zig:13:24: error: redundant comptime keyword in already comptime scope
pub const HkdfSha256 = comptime Hkdf(hmac.sha2.HmacSha256);
                       ^
crypto/hkdf.zig:13:24: error: redundant comptime keyword in already comptime scope
pub const HkdfSha256 = comptime Hkdf(hmac.sha2.HmacSha256);
                       ^

Only 1 file which failed AstGen should have its errors reported, and others silenced.