Add support for gnu_printf format attribute

[llvmbot]

Bugzilla Link	15847
Version	trunk
OS	Windows XP
Blocks	llvm/llvm-bugzilla-archive#15848
Reporter	LLVM Bugzilla Contributor

Extended Description

The gnu_printf format attribute was added in GCC 4.4 by http://gcc.gnu.org/viewcvs/gcc?view=revision&revision=133365.

As Clang defines GNUC, Clang should also support the gnu_printf format attribute.

[llvmbot]

mentioned in issue llvm/llvm-bugzilla-archive#15848

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [15847](https://llvm.org/bz15847) | | Version | trunk | | OS | Windows XP | | Blocks | llvm/llvm-bugzilla-archive#15848 | | Reporter | LLVM Bugzilla Contributor |

Extended Description

The gnu_printf format attribute was added in GCC 4.4 by http://gcc.gnu.org/viewcvs/gcc?view=revision&revision=133365.

As Clang defines GNUC, Clang should also support the gnu_printf format attribute.