Don't warn about selectany on implicitly inline variables

[rnk]

Summary

This avoids a -Wignored-attribute warning on the code pattern Microsoft
recommends for integral const static data members defined in headers
here:
https://docs.microsoft.com/en-us/cpp/build/reference/microsoft-extensions-to-c-and-cpp?view=vs-2019

The attribute is redundant, but it is necessary when compiling in C++14
modes with /Za, which disables MSVC's extension that treats such
variables as implicitly inline.

Fixes PR43270

clang/lib/Sema/SemaDecl.cpp

if (isa<C11NoReturnAttr>(NewAttribute)) {
         S.Diag(Def->getLocation(), diag::err_alignas_missing_on_definition)
           << AA;
         S.Diag(NewAttribute->getLocation(), diag::note_alignas_on_declaration)
           << AA;
         NewAttributes.erase(NewAttributes.begin() + I);
         --E;
         continue;
       }
+    } else if (isa<SelectAnyAttr>(NewAttribute) &&
+               cast<VarDecl>(New)->isInline() &&
+               !cast<VarDecl>(New)->isInlineSpecified()) {
+      // Don't warn about applying selectany to implicitly inline variables.
+      // Older compilers and language modes would require the use of selectany
+      // to make such variables inline, and it would have no effect if we
+      // honored it.
+      ++I;
+      continue;
     }
 
     S.Diag(NewAttribute->getLocation(),
            diag::warn_attribute_precede_definition);
     S.Diag(Def->getLocation(), diag::note_previous_definition);
     NewAttributes.erase(NewAttributes.begin() + I);
     --E;
   }

clang/test/SemaCXX/declspec-selectany.cpp

+// RUN: %clang_cc1 -std=c++14 %s -triple x86_64-windows-msvc -fdeclspec -verify
+// RUN: %clang_cc1 -std=c++17 %s -triple x86_64-windows-msvc -fdeclspec -verify
+// RUN: %clang_cc1 -std=c++14 %s -triple x86_64-scei-ps4 -fdeclspec -verify
+
+// MSVC emits this error too.
+const int __declspec(selectany) test1 = 0; // expected-error {{'selectany' can only be applied to data items with external linkage}}
+
+extern const int test2;
+const int test2 = 42; // expected-note {{previous definition is here}}
+extern __declspec(selectany) const int test2; // expected-warning {{attribute declaration must precede definition}}
+
+extern const int test3;
+const int __declspec(selectany) test3 = 42; // Standard usage.
+
+struct Test4 {
+  static constexpr int sdm = 0;
+};
+__declspec(selectany) constexpr int Test4::sdm; // no warning