Enable __declspec(selectany) on linux

This feature was disabled probably by mistake in rL300562
This fixes bug https://bugs.llvm.org/show_bug.cgi?id=33285

Patch-Source: https://reviews.llvm.org/D33852
See-Also: https://reviews.llvm.org/rL313278
See-Also: https://github.com/alpinelinux/aports/pull/2342#issuecomment-341971965

--- a/include/clang/Basic/Attr.td
+++ b/include/clang/Basic/Attr.td
@@ -2472,9 +2472,9 @@
   let Documentation = [DLLImportDocs];
 }
 
-def SelectAny : InheritableAttr, TargetSpecificAttr<TargetWindows> {
+def SelectAny : InheritableAttr {
   let Spellings = [Declspec<"selectany">, GCC<"selectany">];
-  let Documentation = [Undocumented];
+  let Documentation = [SelectAnyDocs];
 }
 
 def Thread : Attr {
--- a/include/clang/Basic/AttrDocs.td
+++ b/include/clang/Basic/AttrDocs.td
@@ -3192,3 +3192,18 @@
 ensure that this class cannot be subclassed.
   }];
 }
+
+
+def SelectAnyDocs : Documentation {
+  let Category = DocCatType;
+  let Content = [{
+This attribute appertains to a global symbol, causing it to have a weak
+definition (
+`linkonce <https://llvm.org/docs/LangRef.html#linkage-types>`_
+), allowing the linker to select any definition.
+
+For more information see
+`gcc documentation <https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Microsoft-Windows-Variable-Attributes.html>`_
+or `msvc documentation <https://docs.microsoft.com/pl-pl/cpp/cpp/selectany>`_.
+}];
+}
--- a/test/Sema/attr-selectany.c
+++ b/test/Sema/attr-selectany.c
@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -triple x86_64-win32 -fdeclspec -verify %s
 // RUN: %clang_cc1 -triple x86_64-mingw32 -verify %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux -verify -fdeclspec %s
+// RUN: %clang_cc1 -triple x86_64-win32-macho -verify -fdeclspec %s
 
 extern __declspec(selectany) const int x1 = 1; // no warning, const means we need extern in C++
 
--- a/test/SemaCXX/attr-selectany.cpp
+++ b/test/SemaCXX/attr-selectany.cpp
@@ -1,4 +1,7 @@
 // RUN: %clang_cc1 -triple x86_64-win32 -fms-compatibility -fms-extensions -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -triple x86_64-unknown-linux -fms-compatibility -fms-extensions -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -triple x86_64-win32-macho -fms-compatibility -fms-extensions -fsyntax-only -verify -std=c++11 %s
+
 // MSVC produces similar diagnostics.
 
 __declspec(selectany) void foo() { } // expected-error{{'selectany' can only be applied to data items with external linkage}}