Fix CI: switch to --std=c++20

Author: simonmar

glean.cabal.in

@@ -56,7 +56,7 @@ common fb-haskell
        ghc-options: -O2
 
 common fb-cpp
-  cxx-options: -std=c++17 -Wno-nullability-completeness -fno-omit-frame-pointer
+  cxx-options: -std=c++20 -Wno-nullability-completeness -fno-omit-frame-pointer
   if !flag(clang)
      cxx-options: -fcoroutines
   if flag(asan)
@@ -202,7 +202,7 @@ common deps
         hinotify ^>= 0.4.1
 
 common hsc2hs-cpp
-    hsc2hs-options: --cc=g++ --lflag=-lstdc++ --cflag=-D__HSC2HS__=1 --cflag=-std=c++17
+    hsc2hs-options: --cc=g++ --lflag=-lstdc++ --cflag=-D__HSC2HS__=1 --cflag=-std=c++20
 
 common thrift-server
     if flag(fbthrift)

glean/lang/clang/glean-clang.cabal

@@ -103,7 +103,7 @@ common fb-haskell
       ghc-options: -O2
 
 common fb-cpp
-  cxx-options: -DOSS=1 -std=c++17 -Wno-nullability-completeness
+  cxx-options: -DOSS=1 -std=c++20 -Wno-nullability-completeness
   if arch(x86_64)
       cxx-options: -DGLEAN_X86_64 -march=haswell
   if flag(opt)
@@ -142,11 +142,7 @@ executable clang-index
         gflags,
         atomic,
         re2
-    cxx-options: -fexceptions -DOSS=1 -std=c++17
-    if arch(x86_64)
-      cxx-options: -DGLEAN_X86_64 -march=haswell
-    if flag(opt)
-      cxx-options: -O3
+    cxx-options: -fexceptions
 
 executable clang-derive
     import: fb-haskell, fb-cpp, deps, exe