Fix bug in readability-uppercase-literal-suffix

Fixes https://bugs.llvm.org/show_bug.cgi?id=51790. The check triggers
incorrectly with non-type template parameters.

A bisect determined that the bug was introduced here:
ea2225a10b

Unfortunately that patch can no longer be reverted on top of the main
branch, so add a fix instead. Add a unit test to avoid regression in
the future.

clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp

@@ -134,6 +134,11 @@ shouldReplaceLiteralSuffix(const Expr &Literal,
       CharSourceRange::getTokenRange(*Range), SM, LO, &Invalid);
   assert(!Invalid && "Failed to retrieve the source text.");
 
+  // Make sure the first character is actually a digit, instead of
+  // something else, like a non-type template parameter.
+  if (!std::isdigit(static_cast<unsigned char>(LiteralSourceText.front())))
+    return llvm::None;
+
   size_t Skip = 0;
 
   // Do we need to ignore something before actually looking for the suffix?

clang-tools-extra/test/clang-tidy/checkers/readability-uppercase-literal-suffix-integer.cpp

@@ -270,3 +270,29 @@ void c() { l &a(); }
 void d();
 void d() { c<b>(); }
 } // namespace
+
+// Check that non-type template parameters do not cause any diags.
+// https://bugs.llvm.org/show_bug.cgi?id=51790
+template <int capacity>
+struct Vector {
+  static constexpr int kCapacity = capacity;
+};
+
+template <int capacity>
+constexpr int Vector<capacity>::kCapacity;
+// CHECK-MESSAGES-NOT: :[[@LINE-1]]:22: warning: integer literal has suffix 'ity', which is not uppercase
+
+template <int foo1u>
+struct Foo {
+  static constexpr int kFoo = foo1u;
+};
+
+template <int foo1u>
+constexpr int Foo<foo1u>::kFoo;
+// CHECK-MESSAGES-NOT: :[[@LINE-1]]:19: warning: integer literal has suffix 'u', which is not uppercase
+
+// The template needs to be instantiated for diagnostics to show up
+void test_non_type_template_parameter() {
+  int x = Vector<10>::kCapacity;
+  int f = Foo<10>::kFoo;
+}