Support for groups of attributes in #pragma clang attribute

This commit enables pushing an empty #pragma clang attribute push, then adding
multiple attributes to it, then popping them all with #pragma clang attribute
pop, just like #pragma clang diagnostic. We still support the current way of
adding these, #pragma clang attribute push(__attribute__((...))), by treating it
like a combined push/attribute. This is needed to create macros like:

DO_SOMETHING_BEGIN(attr1, attr2, attr3)
// ...
DO_SOMETHING_END

rdar://45496947

Differential revision: https://reviews.llvm.org/D53621

llvm-svn: 345486