Build: Bump build mode to C++20.

Author: sashabu

.bazelrc

@@ -1,3 +1,3 @@
-build --repo_env=CC=clang --repo_env=CXX=clang++ --cxxopt="-std=c++17"
+build --repo_env=CC=clang --repo_env=CXX=clang++ --cxxopt="-std=c++20"
 
 try-import %workspace%/local.bazelrc

swift/.clang-format

@@ -4,4 +4,4 @@ IndentWidth: 2
 SortIncludes: false
 AllowShortIfStatementsOnASingleLine: WithoutElse
 AlwaysBreakBeforeMultilineStrings: false
-Standard: c++17
+Standard: c++20

swift/extractor/infra/file/BUILD.bazel

@@ -24,7 +24,7 @@ genrule(
     # see if https://cplusplus.github.io/LWG/issue3657 is fixed with the current compiler or not
     # if fixed, PathHash.h.workaround will not compile
     cmd = "\n".join([
-        "if clang -c -x c++ -std=c++17 -Wno-pragma-once-outside-header \\",
+        "if clang -c -x c++ -std=c++20 -Wno-pragma-once-outside-header \\",
         "    $(rootpath PathHash.h.workaround) -o /dev/null &> /dev/null; then",
         "  cp $(rootpath PathHash.h.workaround) $@",
         "else",