[ASTMatchers] add a matcher for static locals

[jranieri-grammatech]

Summary

This adds a matcher, isStaticLocal, that matches local static variables (i.e. VarDecl::isStaticLocal).

[aaron.ballman]

LGTM! Do you need someone to commit on your behalf?

[jranieri-grammatech]

I do.

[aaron.ballman]

Committed in r345502, thank you for the patch!

docs/LibASTMatchersReference.html

Given:
   int z = 3;
 functionDecl(isExternC())
   matches the declaration of f and g, but not the declaration of h.
 varDecl(isExternC())
   matches the declaration of x and y, but not the declaration of z.
 </pre></td></tr>
 
 
+<tr><td>Matcher&lt;<a href="http://clang.llvm.org/doxygen/classclang_1_1VarDecl.html">VarDecl</a>&gt;</td><td class="name" onclick="toggle('isStaticLocal0')"><a name="isStaticLocal0Anchor">isStaticLocal</a></td><td></td></tr>
+<tr><td colspan="4" class="doc" id="isStaticLocal0"><pre>Matches a static variable with local scope.
+
+Example matches y (matcher = varDecl(isStaticLocal()))
+void f() {
+  int x;
+  static int y;
+}
+static int z;
+</pre></td></tr>
+
+
 <tr><td>Matcher&lt;<a href="http://clang.llvm.org/doxygen/classclang_1_1VarDecl.html">VarDecl</a>&gt;</td><td class="name" onclick="toggle('isStaticStorageClass1')"><a name="isStaticStorageClass1Anchor">isStaticStorageClass</a></td><td></td></tr>
 <tr><td colspan="4" class="doc" id="isStaticStorageClass1"><pre>Matches variablefunction declarations that have "static" storage
 class specifier ("static" keyword) written in the source.
 
 Given:
   static void f() {}
   static int i = 0;
   extern int j;

include/clang/ASTMatchers/ASTMatchers.h

 AST_MATCHER_P(
     VarDecl, hasInitializer, internal::Matcher<Expr>,
     InnerMatcher) {
   const Expr *Initializer = Node.getAnyInitializer();
   return (Initializer != nullptr &&
           InnerMatcher.matches(*Initializer, Finder, Builder));
 }
 
+/// \brief Matches a static variable with local scope.
+///
+/// Example matches y (matcher = varDecl(isStaticLocal()))
+/// \code
+/// void f() {
+///   int x;
+///   static int y;
+/// }
+/// static int z;
+/// \endcode
+AST_MATCHER(VarDecl, isStaticLocal) {
+  return Node.isStaticLocal();
+}
+
 /// Matches a variable declaration that has function scope and is a
 /// non-static local variable.
 ///
 /// Example matches x (matcher = varDecl(hasLocalStorage())
 /// \code
 /// void f() {
 ///   int x;
 ///   static int y;

lib/ASTMatchers/Dynamic/Registry.cpp

RegistryMaps::RegistryMaps() {
   REGISTER_MATCHER(isNoThrow);
   REGISTER_MATCHER(isOverride);
   REGISTER_MATCHER(isPrivate);
   REGISTER_MATCHER(isProtected);
   REGISTER_MATCHER(isPublic);
   REGISTER_MATCHER(isPure);
   REGISTER_MATCHER(isScoped);
   REGISTER_MATCHER(isSignedInteger);
+  REGISTER_MATCHER(isStaticLocal);
   REGISTER_MATCHER(isStaticStorageClass);
   REGISTER_MATCHER(isStruct);
   REGISTER_MATCHER(isTemplateInstantiation);
   REGISTER_MATCHER(isTypeDependent);
   REGISTER_MATCHER(isUnion);
   REGISTER_MATCHER(isUnsignedInteger);
   REGISTER_MATCHER(isUserProvided);
   REGISTER_MATCHER(isValueDependent);

unittests/ASTMatchers/ASTMatchersNarrowingTest.cpp

TEST(Matcher, VarDecl_Storage) {
   EXPECT_TRUE(notMatches("void f() { static int X; }", M));
 
   M = varDecl(hasName("X"), hasGlobalStorage());
   EXPECT_TRUE(notMatches("void f() { int X; }", M));
   EXPECT_TRUE(matches("int X;", M));
   EXPECT_TRUE(matches("void f() { static int X; }", M));
 }
 
+TEST(Matcher, VarDecl_IsStaticLocal) {
+  auto M = varDecl(isStaticLocal());
+  EXPECT_TRUE(matches("void f() { static int X; }", M));
+  EXPECT_TRUE(notMatches("static int X;", M));
+  EXPECT_TRUE(notMatches("void f() { int X; }", M));
+  EXPECT_TRUE(notMatches("int X;", M));
+}
+
 TEST(Matcher, VarDecl_StorageDuration) {
   std::string T =
     "void f() { int x; static int y; } int a;static int b;extern int c;";
 
   EXPECT_TRUE(matches(T, varDecl(hasName("x"), hasAutomaticStorageDuration())));
   EXPECT_TRUE(
     notMatches(T, varDecl(hasName("y"), hasAutomaticStorageDuration())));
   EXPECT_TRUE(