genTryCpp to catch by Nim type, ready for first review (#7196)

* Rewrite genTryCpp

* correction

* Implement polymorphic raise in cpp

* revert backticks in emit

* Cleanp a comment

* revert test changes

* better handling of <new> header

tests/exception/tdont_overwrite_typename.nim

@@ -1,4 +1,5 @@
 discard """
+  targets: "c cpp"
   output: '''Check passed
 Check passed'''
 """

tests/exception/texcas.nim

@@ -1,4 +1,5 @@
 discard """
+  targets: "c cpp"
   output: '''Hello
 Hello
   '''

tests/exception/tnestedreturn.nim

@@ -1,4 +1,5 @@
 discard """
+  targets: "c cpp"
   file: "tnestedreturn.nim"
   output: "A\nB\nC\n"
 """