[clang-tidy] Skip declarations in system headers in RenamerClangTidyCheck

One typically only wants to perform renaming operations in user code,
not in system headers (which are out of the user's control).
Let's skip those altogether.

This leads to performance improvements in clang-tidy. As a benchmark,
I run all checks on a .cpp file that #includes all C++ standard
headers.

On trunk:

Suppressed 213362 warnings (213362 in non-user code).
Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.

real	0m14.517s
user	0m14.383s
sys	0m0.132s

On this patch:

Suppressed 75107 warnings (75107 in non-user code).

real	0m12.545s
user	0m12.349s
sys	0m0.188s

Author: Carlos Gálvez

clang-tools-extra/clang-tidy/utils/RenamerClangTidyCheck.cpp

@@ -455,6 +455,9 @@ RenamerClangTidyCheck::addUsage(
 void RenamerClangTidyCheck::addUsage(const NamedDecl *Decl,
                                      SourceRange UsageRange,
                                      const SourceManager &SourceMgr) {
+  if (SourceMgr.isInSystemHeader(Decl->getLocation()))
+    return;
+
   if (hasNoName(Decl))
     return;
 

clang-tools-extra/docs/ReleaseNotes.rst

@@ -130,6 +130,10 @@ Changes in existing checks
   <clang-tidy/checks/bugprone/infinite-loop>` check by adding detection for
   variables introduced by structured bindings.
 
+- Improved :doc:`bugprone-reserved-identifier
+  <clang-tidy/checks/bugprone/reserved-identifier>` check by ignoring
+  declarations in system headers.
+
 - Improved :doc:`bugprone-signed-char-misuse
   <clang-tidy/checks/bugprone/signed-char-misuse>` check by fixing
   false positives on C23 enums with the fixed underlying type of signed char.
@@ -160,6 +164,10 @@ Changes in existing checks
   <clang-tidy/checks/portability/template-virtual-member-function>` check to
   avoid false positives on pure virtual member functions.
 
+- Improved :doc:`readability-identifier-naming
+  <clang-tidy/checks/readability/identifier-naming>` check by ignoring
+  declarations in system headers.
+
 - Improved :doc:`readability-qualified-auto
   <clang-tidy/checks/readability/qualified-auto>` check by adding the option
   `IgnoreAliasing`, that allows not looking at underlying types of type aliases.