[lldb] Fixed UB in CPlusPlusLanguage plug-in (#158304)

C++11 allows the use of Universal Character Names (UCNs) in identifiers,
including function names.

According to the spec the behavior of std::isalpha(ch) and
std::isalnum(ch) is undefined if the argument's value is neither
representable as unsigned char nor equal to EOF. To use these functions
safely with plain chars (or signed chars), the argument should first be
converted to unsigned char.

Author: slydiman

lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp

@@ -190,14 +190,16 @@ static bool IsTrivialBasename(const llvm::StringRef &basename) {
   if (basename.size() <= idx)
     return false; // Empty string or "~"
 
-  if (!std::isalpha(basename[idx]) && basename[idx] != '_')
+  if (!std::isalpha(static_cast<unsigned char>(basename[idx])) &&
+      basename[idx] != '_')
     return false; // First character (after removing the possible '~'') isn't in
                   // [A-Za-z_]
 
   // Read all characters matching [A-Za-z_0-9]
   ++idx;
   while (idx < basename.size()) {
-    if (!std::isalnum(basename[idx]) && basename[idx] != '_')
+    if (!std::isalnum(static_cast<unsigned char>(basename[idx])) &&
+        basename[idx] != '_')
       break;
     ++idx;
   }

lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp

@@ -40,8 +40,10 @@ using namespace lldb_private::formatters;
 static void consumeInlineNamespace(llvm::StringRef &name) {
   // Delete past an inline namespace, if any: __[a-zA-Z0-9_]+::
   auto scratch = name;
-  if (scratch.consume_front("__") && std::isalnum(scratch[0])) {
-    scratch = scratch.drop_while([](char c) { return std::isalnum(c); });
+  if (scratch.consume_front("__") &&
+      std::isalnum(static_cast<unsigned char>(scratch[0]))) {
+    scratch = scratch.drop_while(
+        [](char c) { return std::isalnum(static_cast<unsigned char>(c)); });
     if (scratch.consume_front("::")) {
       // Successfully consumed a namespace.
       name = scratch;