Support trailing commas in types within expressions

Author: calda

lib/Parse/ParseType.cpp

@@ -1609,7 +1609,8 @@ bool Parser::canParseGenericArguments() {
     if (!canParseType())
       return false;
     // Parse the comma, if the list continues.
-  } while (consumeIf(tok::comma));
+    // This could be the trailing comma.
+  } while (consumeIf(tok::comma) && !startsWithGreater(Tok));
 
   if (!startsWithGreater(Tok)) {
     return false;

test/Parse/trailing-comma.swift

@@ -102,7 +102,7 @@ struct Foo {
 
 }
 
-func f(in: @differentiable(reverse,) (Int) -> Int) { } // expected-warning {{@differentiable' has been renamed to '@differentiable(reverse)' and will be removed in the next release}} expected-error {{unexpected ',' separator}} expected-error {{expected ',' separator}} expected-error {{unnamed parameters must be written with the empty name '_'}} 
+func f(in: @differentiable(reverse,) (Int) -> Int) { } // expected-warning {{@differentiable' has been renamed to '@differentiable(reverse)' and will be removed in the next release}} expected-error {{expected ',' separator}} expected-error {{unnamed parameters must be written with the empty name '_'}}
 
 @derivative(of: Self.other,) // expected-error {{unexpected ',' separator}}
 func foo() {}
@@ -144,7 +144,7 @@ while true, { } // expected-error {{expected '{' after 'while' condition}}
 if #available(OSX 51,) { // expected-error {{expected platform name}}
 }
 
-@available(OSX 10.7, iOS 7.0, *,) // expected-error {{expected platform name}}
+@available(OSX 10.7, iOS 7.0, *,) // expected-error {{expected platform name}} expected-error {{expected declaration}}
 @_originallyDefinedIn(module: "HighLevel", OSX 10.9, iOS 13.0,) // expected-error {{unexpected ',' separator}}
-@backDeployed(before: OSX 10.9,) // expected-error {{expected version number in '@backDeployed' attribute}}
+@backDeployed(before: OSX 10.9,) // expected-error {{unexpected ',' separator}}
 public struct StructWithAvailability {}

test/decl/ext/typealias.swift

@@ -88,3 +88,6 @@ typealias Bar2<
   T2,
   Bool,
 >
+
+let _ = Foo2<Int, Bool, String,>.self
+let _ = Bar2<Int, Bool,>()

test/type/types.swift

@@ -225,6 +225,8 @@ let tupleTypeWithTrailingComma: (
   quux: String,
 )
 
+let _ = (bar: String, quux: String,).self
+
 let closureTypeWithTrailingCommas: (
   String,
   String,