Bug 1468811 - Re-enable clang-tidy check performance-faster-string-find. r=sylvestre

It already had test files, but wasn't actually enabled. Also add a mock internal
std::string::find signature; two comments about disabled checks; and fix a typo.

tools/clang-tidy/config.yaml

@@ -17,6 +17,7 @@ clang_checkers:
     publish: !!bool yes
   - name: clang-analyzer-security.insecureAPI.getpw
     publish: !!bool yes
+  # We don't add clang-analyzer-security.insecureAPI.gets here; it's deprecated.
   - name: clang-analyzer-security.insecureAPI.mkstemp
     publish: !!bool yes
   - name: clang-analyzer-security.insecureAPI.mktemp
@@ -72,8 +73,13 @@ clang_checkers:
     publish: !!bool no
   - name: mozilla-*
     publish: !!bool yes
+  - name: performance-faster-string-find
+    publish: !!bool yes
   - name: performance-for-range-copy
     publish: !!bool yes
+  # Only available from clang tidy 6.0. We are currently using 5.0
+  # - name: performance-implicit-conversion-in-loop
+  #   publish: !!bool yes
   - name: performance-inefficient-string-concatenation
     publish: !!bool yes
   - name: performance-inefficient-vector-operation

tools/clang-tidy/test/performance-faster-string-find.cpp

@@ -1,4 +1,4 @@
-#include <string>
+#include "structures.h"
 
 void foo() {
   std::string str;

tools/clang-tidy/test/structures.h

@@ -1,4 +1,4 @@
-// Proxy file in order to define generic data types, to void binding with system headers
+// Proxy file in order to define generic data types, to avoid binding with system headers
 
 namespace std {
 
@@ -41,6 +41,7 @@ public:
   ~basic_string() {}
   size_t size() const;
   bool empty() const;
+  size_t find (const char* s, size_t pos = 0) const;
   const T *c_str() const;
   _Type& assign(const T *s);
   basic_string<T> *operator+=(const basic_string<T> &) {}