fixes #25803: add genCppConstructorExpr for full type-prefixed expression (#25817)

fixes #25803

This pull request introduces a new approach for generating C++
constructor expressions in the Nim compiler's C++ backend, ensuring that
type-prefixed construction is used when needed (such as in assignments),
rather than just braced initializer lists. It also adds a new test case
(with corresponding C++ header) to verify correct behavior when
assigning to types with overloaded assignment operators and
constructors.

**C++ code generation improvements:**

* Added `genCppConstructorExpr` in `ccgtypes.nim`, which generates a
full type-prefixed constructor expression (e.g., `Foo(a, b)`) for C++
code generation, as opposed to just a braced initializer list. This is
important for contexts like assignments where the type must be explicit.
* Updated `resetLoc` in `cgen.nim` to use `genCppConstructorExpr`
instead of `genCppInitializer` when initializing imported C++ types,
ensuring correct code generation for assignments.

**Testing:**

* Added a new C++ header file, `tcpp_default_ctor_assignment.h`,
defining a struct `AmbiguousAssign` with overloaded assignment operators
and constructors to test ambiguous assignment scenarios.
* Added a corresponding Nim test, `tcpp_default_ctor_assignment.nim`,
which exercises construction and assignment for the imported C++ type,
ensuring the new code generation logic works as intended.

tests/cpp/tcpp_default_ctor_assignment.h

@@ -0,0 +1,30 @@
+#ifndef TCPP_DEFAULT_CTOR_ASSIGNMENT_H
+#define TCPP_DEFAULT_CTOR_ASSIGNMENT_H
+
+struct AmbiguousAssign {
+  int x;
+  const char* y;
+
+  AmbiguousAssign(): x(0), y(nullptr) {}
+  AmbiguousAssign(int x, const char* y): x(x), y(y) {}
+
+  AmbiguousAssign& operator=(int v) {
+    x = v;
+    y = nullptr;
+    return *this;
+  }
+
+  AmbiguousAssign& operator=(const char* s) {
+    x = 0;
+    y = s;
+    return *this;
+  }
+
+  AmbiguousAssign& operator=(const AmbiguousAssign& other) {
+    x = other.x;
+    y = other.y;
+    return *this;
+  }
+};
+
+#endif

tests/cpp/tcpp_default_ctor_assignment.nim

@@ -0,0 +1,14 @@
+discard """
+  cmd: "nim cpp $file"
+"""
+
+type
+  AmbiguousAssign {.importcpp, header: "tcpp_default_ctor_assignment.h".} = object
+    x: cint
+    y: cstring
+
+proc main =
+  var xs = newSeq[AmbiguousAssign](3)
+  doAssert xs.len == 3
+
+main()