[ruby/prism] Fix parser translator when splatting in pattern matching pin

Because it ends up treating it as a local variable, and `a.x`
is not a valid local variable name.

I'm not big on pattern matching, but conceptually it makes sense to me
to treat anything inside ^() to not be
pattern matching syntax?

https://github.com/ruby/prism/commit/80dbd85c45

lib/prism/translation/parser/compiler.rb

@@ -1481,7 +1481,8 @@ module Prism
         # foo => ^(bar)
         #        ^^^^^^
         def visit_pinned_expression_node(node)
-          expression = builder.begin(token(node.lparen_loc), visit(node.expression), token(node.rparen_loc))
+          parts = node.expression.accept(copy_compiler(in_pattern: false)) # Don't treat * and similar as match_rest
+          expression = builder.begin(token(node.lparen_loc), parts, token(node.rparen_loc))
           builder.pin(token(node.operator_loc), expression)
         end
 

test/prism/fixtures/patterns.txt

@@ -218,3 +218,7 @@ a in b, and c
 a in b, or c
 (a in b,) and c
 (a in b,) or c
+
+x => ^([*a.x])
+x => ^([**a.x])
+x => ^({ a: })