clang: Add specificity to libstdc++ workaround

[urnathan]

Summary

Here's an alternative to https://reviews.llvm.org/D100469. Add some specificity to the comment, so future-someone has an easier time telling that it is no longer needed.

[rsmith]

clang/lib/Sema/SemaDeclCXX.cpp

12138–12140

Utterly trivial style nits: we typically use only one space after a period and end comments with a period.

[urnathan]

057b6f5d0b3e 2021-04-16 | clang: Update libstdc++ issue workaround

clang/lib/Sema/SemaDeclCXX.cpp

   LookupQualifiedName(R, LookupContext);
 
   // Try to correct typos if possible. If constructor name lookup finds no
   // results, that means the named class has no explicit constructors, and we
   // suppressed declaring implicit ones (probably because it's dependent or
   // invalid).
   if (R.empty() &&
       NameInfo.getName().getNameKind() != DeclarationName::CXXConstructorName) {
-    // HACK: Work around a bug in libstdc++'s detection of ::gets. Sometimes
-    // it will believe that glibc provides a ::gets in cases where it does not,
-    // and will try to pull it into namespace std with a using-declaration.
-    // Just ignore the using-declaration in that case.
+    // HACK 2017-01-08: Work around an issue with libstdc++'s detection of
+    // ::gets.  Sometimes it believes that glibc provides a ::gets in cases
+    // where it does not.  The issue was fixed in libstdc++ 6.3 (2016-12-21) and
+    // later
     auto *II = NameInfo.getName().getAsIdentifierInfo();
     if (getLangOpts().CPlusPlus14 && II && II->isStr("gets") &&
         CurContext->isStdNamespace() &&
         isa<TranslationUnitDecl>(LookupContext) &&
         getSourceManager().isInSystemHeader(UsingLoc))
       return nullptr;
     UsingValidatorCCC CCC(HasTypenameKeyword, IsInstantiation, SS.getScopeRep(),
                           dyn_cast<CXXRecordDecl>(CurContext));