typechecker: support Enum in xhp attribute declarations

Reviewed By: @int3

Differential Revision: D1758269

Signature: t1:1758269:1419908206:4f7d0f79356d4225d45244c8d6bb22eb1f1d69bf
Make multiple inheritance a parse error

Reviewed By: @elgenie

Differential Revision: D1760023

Signature: t1:1760023:1419913001:9b1239f87d93b721329d75bfada37827f8d59a4d
Make `trait ... extends ...` into a parse error

Reviewed By: @elgenie

Differential Revision: D1760037

Signature: t1:1760037:1419901068:a37f69069762dcd3f0259c034f166244236f1163