add doc and test

Author: HerrCai0907

clang/docs/LibASTMatchersReference.html

@@ -804,6 +804,17 @@ <h2 id="decl-matchers">Node Matchers</h2>
 </pre></td></tr>
 
 
+<tr><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1Decl.html">Decl</a>&gt;</td><td class="name" onclick="toggle('exportDecl0')"><a name="exportDecl0Anchor">exportDecl</a></td><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1ExportDecl.html">ExportDecl</a>&gt;...</td></tr>
+<tr><td colspan="4" class="doc" id="exportDecl0"><pre>Matches any export declaration.
+
+Example matches following declarations.
+  export void foo();
+  export { void foo(); }
+  export namespace { void foo(); }
+  export int v;
+</pre></td></tr>
+
+
 <tr><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1Decl.html">Decl</a>&gt;</td><td class="name" onclick="toggle('fieldDecl0')"><a name="fieldDecl0Anchor">fieldDecl</a></td><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1FieldDecl.html">FieldDecl</a>&gt;...</td></tr>
 <tr><td colspan="4" class="doc" id="fieldDecl0"><pre>Matches field declarations.
 

clang/unittests/ASTMatchers/ASTMatchersNodeTest.cpp

@@ -189,6 +189,21 @@ TEST(ASTMatchersTestCUDA, HasAttrCUDA) {
                                   hasAttr(clang::attr::CUDAGlobal)));
 }
 
+TEST_P(ASTMatchersTest, ExportDecl) {
+  if (!GetParam().isCXX20OrLater()) {
+    return;
+  }
+  const std::string moduleHeader = "module;export module ast_matcher_test;";
+  EXPECT_TRUE(matches(moduleHeader + "export void foo();",
+                      exportDecl(has(functionDecl()))));
+  EXPECT_TRUE(matches(moduleHeader + "export { void foo(); int v; }",
+                      exportDecl(has(functionDecl()))));
+  EXPECT_TRUE(matches(moduleHeader + "export { void foo(); int v; }",
+                      exportDecl(has(varDecl()))));
+  EXPECT_TRUE(matches(moduleHeader + "export namespace aa { void foo(); }",
+                      exportDecl(has(namespaceDecl()))));
+}
+
 TEST_P(ASTMatchersTest, ValueDecl) {
   if (!GetParam().isCXX()) {
     // FIXME: Fix this test in non-C++ language modes.