[R] [C++]: Fix RE2 compilation errors under C++20

[jonkeane]

Describe the enhancement requested

We've received a two-week email from CRAN to fix the errors they are seeing. One appears to be RE2 on Fedora causing issues now that we are requiring C++20:

2. WARNING: Compiler Warnings on Fedora (WARN)
   Affected platform: r-devel-linux-x86_64-fedora-clang Issue: Multiple warnings about anonymous types in anonymous unions in the bundled RE2 library:

Found the following significant warnings:
   [0m [1m/tmp/Rtmp92SHzx/working_dir/RtmpS2Y52i/file3e9f371c38502f/_deps/re2-src/re2/prog.h:148:7:  [0m [0;1;35mwarning:  [0m [1manonymous types declared in an anonymous union are an extension [-Wnested-anon-types] [0m
   [0m [1m/tmp/Rtmp92SHzx/working_dir/RtmpS2Y52i/file3e9f371c38502f/_deps/re2-src/re2/prog.h:426:5:  [0m [0;1;35mwarning:  [0m [1manonymous types declared in an anonymous union are an extension [-Wnested-anon-types] [0m
   [0m [1m/tmp/Rtmp92SHzx/working_dir/RtmpS2Y52i/file3e9f371c38502f/_deps/re2-src/re2/regexp.h:574:5:  [0m [0;1;35mwarning:  [0m [1manonymous types declared in an anonymous union are an extension [-Wnested-anon-types] [0m
   [0m [1m/tmp/Rtmp92SHzx/working_dir/RtmpS2Y52i/file3e9f371c38502f/_deps/re2-src/re2/regexp.h:578:5:  [0m [0;1;35mwarning:  [0m [1manonymous types declared in an anonymous union are an extension [-Wnested-anon-types] [0m
   [0m [1m/tmp/Rtmp92SHzx/working_dir/RtmpS2Y52i/file3e9f371c38502f/_deps/re2-src/re2/regexp.h:582:5:  [0m [0;1;35mwarning:  [0m [1manonymous types declared in an anonymous union are an extension [-Wnested-anon-types] [0m
   [0m [1m/tmp/Rtmp92SHzx/working_dir/RtmpS2Y52i/file3e9f371c38502f/_deps/re2-src/re2/regexp.h:586:5:  [0m [0;1;35mwarning:  [0m [1manonymous types declared in an anonymous union are an extension [-Wnested-anon-types] [0m

Component(s)

C++, R

[jonkeane]

I'm working on getting a CI job to replicate this. Our current setups have a gap where we don't have Fedora + CLANG + libc++