[TableGen] Only normalize the spelling of GNU-style attributes.

Summary:
When Sema looks up an attribute name, it strips off leading and trailing
"__" if the attribute is GNU-style.  That is, __attribute__((foo)) and
__attribute__((__foo__)) are equivalent.

This is only true for GNU-style attributes.  In particular,
__declspec(__foo__) is not equivalent to __declspec(foo), and Sema
respects this difference.

This patch fixes TableGen to match Sema's behavior.  The spelling
'GNU<"__foo__">' should be normalized to 'GNU<"foo">', but
'Declspec<"__foo__">' should not be changed.

This is necessary to make CUDA compilation work on Windows, because e.g.
the __device__ attribute is spelled __declspec(__device__).

Attr.td does not contain any Declspec spellings that start or end with
"__", so this change should not affect any other attributes.

Reviewers: rnk

Subscribers: cfe-commits, tra

Differential Revision: https://reviews.llvm.org/D28318

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@291129 91177308-0d34-0410-b5e6-96231b3b80d8