[pseudo] Remove unnecessary user-defined-string-literal rule.

[hokein]

Summary

We accidently define two identical rules for user-defined-string-literal
(one left recursive and the other one is right recursive), it explodes
states when parsing a long long "" "" "" string literal.

TEST on a huge file which contains ~2.6w "" string literal:

before this patch: more than minutes
after this patch: < 1s

[hokein]

clang-tools-extra/pseudo/lib/cxx.bnf

716

I also wrote this as right recursive, it significantly reduces the states in the GSS (35% saving!)

before:

Forest bytes: 1898128 nodes: 105456
GSS bytes: 3801192 nodes: 158029

after:

Forest bytes: 1898128 nodes: 105456
GSS bytes: 2539624 nodes: 105433

[sammccall]

The intent here is that user-defined-string-literal-chunk := STRING_LITERAL only matches when there's a ud-suffix. And string-literal-chunk := STRING_LITERAL only matches when there isn't.
(e.g. with a rule guard, which we don't have implemented)
If this was happening, would we still have the explosion?

I believe (once the guards are added) this patch will lead to rejecting "foo" "bar"_ud "baz".

clang-tools-extra/pseudo/lib/cxx.bnf

 string-literal-chunk := WIDE_STRING_LITERAL
 string-literal-chunk := UTF8_STRING_LITERAL
 string-literal-chunk := UTF16_STRING_LITERAL
 string-literal-chunk := UTF32_STRING_LITERAL
 #! Technically, string concatenation happens at phase 6 which is before parsing,
 #! so it doesn't belong to the grammar. However, we extend the grammar to
 #! support it, to make the pseudoparser fully functional on practical code.
 string-literal := string-literal-chunk
-string-literal := string-literal string-literal-chunk
+string-literal := string-literal-chunk string-literal
 user-defined-literal := user-defined-integer-literal
 user-defined-literal := user-defined-floating-point-literal
 user-defined-literal := user-defined-string-literal
 user-defined-literal := user-defined-character-literal
 user-defined-integer-literal := NUMERIC_CONSTANT
 user-defined-string-literal-chunk := STRING_LITERAL
 user-defined-string-literal-chunk := WIDE_STRING_LITERAL
 user-defined-string-literal-chunk := UTF8_STRING_LITERAL
 user-defined-string-literal-chunk := UTF16_STRING_LITERAL
 user-defined-string-literal-chunk := UTF32_STRING_LITERAL
 user-defined-string-literal := user-defined-string-literal-chunk
 user-defined-string-literal := string-literal-chunk user-defined-string-literal
-user-defined-string-literal := user-defined-string-literal string-literal-chunk
 user-defined-floating-point-literal := NUMERIC_CONSTANT
 user-defined-character-literal := CHAR_CONSTANT
 user-defined-character-literal := WIDE_CHAR_CONSTANT
 user-defined-character-literal := UTF8_CHAR_CONSTANT
 user-defined-character-literal := UTF16_CHAR_CONSTANT
 user-defined-character-literal := UTF32_CHAR_CONSTANT
 boolean-literal := FALSE
 boolean-literal := TRUE