[BUG] Raw string with `{}` doesn't parse

[JohelEGP]

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Title: Raw string with {} doesn't parse.

Minimal reproducer (https://cpp2.godbolt.org/z/aPbMedenM):

main: () = {
  _ = R"({
  })";
}

Commands:

cppfront main.cpp2
clang++18 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -Werror=unused-result -I . main.cpp

Expected result: A well-formed string literal.

Actual result and error:

main.cpp2...
main.cpp2(4,0): error: closing } does not match a prior {
main.cpp2: error: unexpected end of source file