[include-cleaner] Ignore the ParmVarDecl itself in WalkAST.cpp

This fixes a false positive where a ParamVarDecl happend to be the
same name of some C standard symbol and has a global namespace.

```
using A = int(int time); // we suggest <ctime> for the `int time`.
```

Differential Revision: https://reviews.llvm.org/D153330

clang-tools-extra/include-cleaner/lib/WalkAST.cpp

@@ -214,6 +214,10 @@ class ASTWalker : public RecursiveASTVisitor<ASTWalker> {
     return true;
   }
   bool VisitVarDecl(VarDecl *VD) {
+    // Ignore the parameter decl itself (its children were handled elsewhere),
+    // as they don't contribute to the main-file #include.
+    if (llvm::isa<ParmVarDecl>(VD))
+      return true;
     // Mark declaration from definition as it needs type-checking.
     if (VD->isThisDeclarationADefinition())
       report(VD->getLocation(), VD);

clang-tools-extra/include-cleaner/unittests/AnalysisTest.cpp

@@ -91,7 +91,8 @@ TEST_F(WalkUsedTest, Basic) {
   #include "header.h"
   #include "private.h"
 
-  void $bar^bar($private^Private $p^p) {
+  // No reference reported for the Parameter "p".
+  void $bar^bar($private^Private p) {
     $foo^foo();
     std::$vector^vector $vconstructor^$v^v;
     $builtin^__builtin_popcount(1);
@@ -120,7 +121,6 @@ TEST_F(WalkUsedTest, Basic) {
       offsetToProviders(AST, SM),
       UnorderedElementsAre(
           Pair(Code.point("bar"), UnorderedElementsAre(MainFile)),
-          Pair(Code.point("p"), UnorderedElementsAre(MainFile)),
           Pair(Code.point("private"),
                UnorderedElementsAre(PublicFile, PrivateFile)),
           Pair(Code.point("foo"), UnorderedElementsAre(HeaderFile)),