[clang-tidy][NFC] Use C++17 nested namespaces in the clang-tidy folder

Fix applied by running:

run-clang-tidy.py -checks=-*,modernize-concat-nested-namespaces

Differential Revision: https://reviews.llvm.org/D141770
diff --git a/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp b/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
index b5f8d08..70a6ab3 100644
--- a/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
+++ b/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
@@ -14,9 +14,7 @@
 
 using namespace clang::ast_matchers;
 
-namespace clang {
-namespace tidy {
-namespace altera {
+namespace clang::tidy::altera {
 
 void StructPackAlignCheck::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(recordDecl(isStruct(), isDefinition(),
@@ -143,6 +141,4 @@
   Options.store(Opts, "MaxConfiguredAlignment", MaxConfiguredAlignment);
 }
 
-} // namespace altera
-} // namespace tidy
-} // namespace clang
+} // namespace clang::tidy::altera