C++: Add library support for char8_t type.

cpp/ql/src/semmle/code/cpp/Type.qll

@@ -376,6 +376,8 @@ private predicate isIntegralType(@builtintype type, int kind) {
     kind = 43
     or
     kind = 44
+    or
+    kind = 51
   )
 }
 
@@ -993,6 +995,18 @@ class WideCharType extends IntegralType {
   override string getCanonicalQLClass() { result = "WideCharType" }
 }
 
+/**
+ * The C/C++ `char8_t` type.  This is available starting with C++20.
+ * ```
+ * char8_t c8;
+ * ```
+ */
+class Char8Type extends IntegralType {
+  Char8Type() { builtintypes(underlyingElement(this), _, 51, _, _, _) }
+
+  override string getCanonicalQLClass() { result = "Char8Type" }
+}
+
 /**
  * The C/C++ `char16_t` type.  This is available starting with C11 and C++11.
  * ```

cpp/ql/src/semmle/code/cpp/padding/Padding.qll

@@ -74,6 +74,8 @@ abstract class Architecture extends string {
     or
     t instanceof WideCharType and result = wideCharSize()
     or
+    t instanceof Char8Type and result = 8
+    or
     t instanceof Char16Type and result = 16
     or
     t instanceof Char32Type and result = 32