Fix -Wshadow violations found by gcc

Summary: [Folly] Fix `-Wshadow` violations found by gcc.

Reviewed By: meyering

Differential Revision: D16578896

fbshipit-source-id: 3ffe6089ad7b78d9c01d085029b55ec94d6adcff

folly/logging/test/helpers/FatalHelper.cpp

@@ -121,7 +121,7 @@ std::string fbLogFatalCheck() {
 }
 
 struct MyStruct {
-  MyStruct(uint32_t a, uint32_t b) : a(a), b(b) {}
+  MyStruct(uint32_t a_, uint32_t b_) : a(a_), b(b_) {}
   uint32_t a;
   uint32_t b;
 };

folly/test/small_vector_test.cpp

@@ -363,7 +363,7 @@ TEST(small_vector, InsertNontrivial) {
   class TestClass {
    public:
     // explicit TestClass() = default;
-    explicit TestClass(std::string s) : s(s) {}
+    explicit TestClass(std::string s_) : s(s_) {}
     std::string s;
   };
   folly::small_vector<TestClass> v3(5, TestClass("asd"));