d: Add `@simd` and `@simd_clones` attributes to compiler and library
The `@simd` attribute is equivalent to `__attribute__((simd))`, and
`@simd_clones` is a convenience alias to allow specifying whether the
compiler should generated masked or non-masked simd clones.
gcc/d/ChangeLog:
* d-attribs.cc (handle_omp_declare_simd_attribute): New function.
(d_handle_simd_attribute): New function.
(d_langhook_common_attribute_table): Add 'omp declare simd' attribute.
(d_langhook_attribute_table): Add simd attribute.
libphobos/ChangeLog:
* libdruntime/gcc/attributes.d (simd): Define.
gcc/testsuite/ChangeLog:
* gdc.dg/attr_simd1.d: New test.
* gdc.dg/attr_simd2.d: New test.