Handle pub tuple fields in tuple structs

The current implementation will throw a parser error for tuple structs
that contain a pub tuple field. For example,
```rust
struct Foo(pub (u32, u32));
```
is valid Rust, but rust-analyzer will throw a parser error.  This is
because the parens after `pub` is treated as a visibility context.
Allowing a tuple type to follow `pub` in the special case when we are
defining fields in a tuple struct can fix the issue.

crates/syntax/test_data/parser/inline/ok/0196_pub_tuple_field.rast

@@ -0,0 +1,30 @@
+SOURCE_FILE@0..33
+  STRUCT@0..32
+    STRUCT_KW@0..6 "struct"
+    WHITESPACE@6..7 " "
+    NAME@7..15
+      IDENT@7..15 "MyStruct"
+    TUPLE_FIELD_LIST@15..31
+      L_PAREN@15..16 "("
+      TUPLE_FIELD@16..30
+        VISIBILITY@16..19
+          PUB_KW@16..19 "pub"
+        WHITESPACE@19..20 " "
+        TUPLE_TYPE@20..30
+          L_PAREN@20..21 "("
+          PATH_TYPE@21..24
+            PATH@21..24
+              PATH_SEGMENT@21..24
+                NAME_REF@21..24
+                  IDENT@21..24 "u32"
+          COMMA@24..25 ","
+          WHITESPACE@25..26 " "
+          PATH_TYPE@26..29
+            PATH@26..29
+              PATH_SEGMENT@26..29
+                NAME_REF@26..29
+                  IDENT@26..29 "u32"
+          R_PAREN@29..30 ")"
+      R_PAREN@30..31 ")"
+    SEMICOLON@31..32 ";"
+  WHITESPACE@32..33 "\n"

crates/syntax/test_data/parser/inline/ok/0196_pub_tuple_field.rs

@@ -0,0 +1 @@
+struct MyStruct(pub (u32, u32));