[clang][ASTImporter] fix variable inline of CXX17 (#87314)

Fix crash in the testcase from
#75114 (comment)
Forget to set inline of variable declaration would make
`isThisDeclarationADefinition` get incorrect result and didn't get
imported variable. This will lead to a new `VarTemplateDecl` being
created and call `setDescribedVarTemplate` again which produces the
crash.

Co-authored-by: huqizhi <836744285@qq.com>

clang/lib/AST/ASTImporter.cpp

@@ -4541,6 +4541,10 @@ ExpectedDecl ASTNodeImporter::VisitVarDecl(VarDecl *D) {
   ToVar->setQualifierInfo(ToQualifierLoc);
   ToVar->setAccess(D->getAccess());
   ToVar->setLexicalDeclContext(LexicalDC);
+  if (D->isInlineSpecified())
+    ToVar->setInlineSpecified();
+  if (D->isInline())
+    ToVar->setImplicitlyInline();
 
   if (FoundByLookup) {
     auto *Recent = const_cast<VarDecl *>(FoundByLookup->getMostRecentDecl());

clang/unittests/AST/ASTImporterTest.cpp

@@ -5317,6 +5317,34 @@ TEST_P(ASTImporterOptionSpecificTestBase,
   EXPECT_FALSE(ToX);
 }
 
+TEST_P(ASTImporterOptionSpecificTestBase, VarTemplateDeclInlineWithCXX17) {
+  Decl *FromTU = getTuDecl(
+      R"(
+      struct S {
+        template <unsigned> static constexpr bool X = true;
+      };
+      )",
+      Lang_CXX17, "input1.cc");
+  Decl *FromTU2 = getTuDecl(
+      R"(
+      struct S {
+        template <unsigned> static constexpr bool X = true;
+        template <typename T> void get() { X<sizeof(T)>; }
+      };
+      template <typename U> U qvariant_cast(const S &v) { return v.get; }
+      )",
+      Lang_CXX17, "input2.cc");
+  auto *FromX = FirstDeclMatcher<VarTemplateDecl>().match(
+      FromTU, varTemplateDecl(hasName("X")));
+  auto *ToX = Import(FromX, Lang_CXX17);
+  ASSERT_TRUE(ToX);
+  auto *FromX2 = FirstDeclMatcher<VarTemplateDecl>().match(
+      FromTU2, varTemplateDecl(hasName("X")));
+  auto *ToX2 = Import(FromX2, Lang_CXX17);
+  EXPECT_TRUE(ToX2);
+  EXPECT_EQ(ToX, ToX2);
+}
+
 TEST_P(ASTImporterOptionSpecificTestBase, VarTemplateParameterDeclContext) {
   constexpr auto Code =
       R"(