Fix strict aliasing violation from conditional typedef of wchar_t by building entire project as C++ for Unix (#6801)

Enable building CoreCLR as C++ project on Unix

This series of patches fixes the strict aliasing violation from
the conditional typedef of wchar_t in src/pal/inc/pal_char16.h:40

* rename c files to cpp
* modify all cmake files to change .c files to .cpp
* apply c++ linkage to templates

1 files changed, 42 insertions, 0 deletions

diff --git a/src/pal/tests/palsuite/c_runtime/_strlwr/test1/test1.cpp b/src/pal/tests/palsuite/c_runtime/_strlwr/test1/test1.cpp
new file mode 100644
index 0000000000..1c4015e3e2
--- /dev/null
+++ b/src/pal/tests/palsuite/c_runtime/_strlwr/test1/test1.cpp
@@ -0,0 +1,42 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================================
+**
+** Source:  test1.c
+**
+** Purpose: 
+**      Using memcmp, check to see that after changing a string into all lowercase
+**      that it is the lowercase string that was expected.
+**
+**
+**==========================================================================*/
+
+#include <palsuite.h>
+
+int __cdecl main(int argc, char *argv[])
+{
+    char string[] = "aASdff";
+    char checkstr[] = "aasdff";
+    char *ret=NULL;
+
+    /*
+     *  Initialize the PAL and return FAIL if this fails
+     */
+    if (0 != (PAL_Initialize(argc, argv)))
+    {
+        return FAIL;
+    }
+
+    ret = _strlwr(string);
+
+    if (memcmp(ret, checkstr, sizeof(checkstr)) != 0)
+    {
+        Fail ("ERROR: _strlwr returning incorrect value\n"
+                "Expected %s, got %s\n", checkstr, ret);
+    }
+    
+    PAL_Terminate();
+    return PASS;
+}