C++ instr: do not instrument constexpr

Instrumenting constexpr functions violates the constexpr semantics and
there is no straightforward valid way of instrumenting such code
constructs.

Skip their instrumentation altogether for the moment, as their use is
quite limited (they are only allowed to return literal values).

Author: Jugst3r

testsuite/tests/C++/mcdc/ConstExpr/src/test_constexpr.cpp

@@ -0,0 +1,28 @@
+constexpr int
+foo ()
+{
+  if (true && false)       // # ce-body
+    return 1;              // # ce-body
+  constexpr bool a = true; // # ce-body
+  if constexpr (false)     // # ce-body
+    return 1;              // # ce-body
+  return 0;                // # ce-body
+}
+
+int
+main(){
+  constexpr bool a = true || false;                    // # single-ce-decl
+  constexpr bool b = true || false, c = true || false; // # double-ce-decl
+  if constexpr (false)                                 // # if-ce
+    return 1;                                          // # if-rt
+  return 0;                                            // # rt
+}
+
+//# test_constexpr.cpp
+//
+//    /ce-body/        l? ## s?
+//    /single-ce-decl/ l? ## d?
+//    /double-ce-decl/ l? ## d?, d?
+//    /if-ce/          l? ## d?
+//    /if-rt/          l- ## s-
+//    /rt/             l+ ## 0

testsuite/tests/C++/mcdc/ConstExpr/test.py

@@ -0,0 +1,5 @@
+from SCOV.tc import TestCase
+from SUITE.context import thistest
+
+TestCase(tolerate_messages=r".* cannot instrument constexpr").run()
+thistest.result()

tools/gnatcov/clang-extensions.adb

@@ -90,6 +90,20 @@ package body Clang.Extensions is
       return Opcode_Str;
    end Get_Opcode_Str;
 
+   ------------------
+   -- Is_Constexpr --
+   ------------------
+
+   function Is_Constexpr (C : Cursor_T) return Boolean
+   is
+      function Is_Constexpr_C (C : Cursor_T) return unsigned
+        with
+          Import, Convention => C,
+          External_Name => "clang_isConstexpr";
+   begin
+      return Is_Constexpr_C (C) /= 0;
+   end Is_Constexpr;
+
    -----------------------------------
    -- CX_Rewriter_Insert_Text_After --
    -----------------------------------

tools/gnatcov/clang-extensions.ads

@@ -88,6 +88,8 @@ package Clang.Extensions is
 
    function Get_Callee_Name_Str (C : Cursor_T) return String with Inline;
 
+   function Is_Constexpr (C : Cursor_T) return Boolean with Inline;
+
    function Unwrap (C : Cursor_T) return Cursor_T
      with Import, Convention => C, External_Name => "clang_unwrap";