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Implement support for GCC __attribute__.

- Implement "TODO" in Parser::ParseAttributes. Changed the return type from
void to Parser::DeclTy. Changed all call sites to accept the return value.
- Added Action::ParseAttribute and Sema::ParseAttribute to return an
appropriate AST node. Added new node AttributeDecl to Decl.h.

Still to do...hook up to the Decl...

llvm-svn: 39539