[nullability] Smart pointers: Implement `weak_ptr::lock()`.

This is the one `weak_ptr` operation that we do want to implement, as we want
the analysis to "know" that the return value of `lock()` is nullable and
therefore needs to be checked.

PiperOrigin-RevId: 590464166
Change-Id: I48c5f43ca235273e82f425ec0e966d0fd8136e11

nullability/pointer_nullability_matchers.cc

diff --git a/nullability/pointer_nullability_matchers.cc b/nullability/pointer_nullability_matchers.cc
index 2006237..dc75724 100644
--- a/nullability/pointer_nullability_matchers.cc
+++ b/nullability/pointer_nullability_matchers.cc
@@ -26,6 +26,7 @@
 using ast_matchers::cxxMemberCallExpr;
 using ast_matchers::cxxMethodDecl;
 using ast_matchers::cxxOperatorCallExpr;
+using ast_matchers::cxxRecordDecl;
 using ast_matchers::cxxThisExpr;
 using ast_matchers::decl;
 using ast_matchers::expr;
@@ -159,6 +160,12 @@
                            hasType(isNullPtrType()))));
 }
 
+Matcher<Stmt> isWeakPtrLockCall() {
+  return cxxMemberCallExpr(
+      thisPointerType(cxxRecordDecl(isInStdNamespace(), hasName("weak_ptr"))),
+      callee(cxxMethodDecl(hasName("lock"))));
+}
+
 Matcher<Stmt> isSupportedPointerAccessorCall() {
   return cxxMemberCallExpr(callee(cxxMethodDecl(hasBody(compoundStmt(
       statementCountIs(1),