travis: upgrade c++ build to focal & llvm 10
travis: upgrade c++ build to focal & llvm 10

ericvergnaud/antlr4,ericvergnaud/antlr4,antlr/antlr4,antlr/antlr4,antlr/antlr4,parrt/antlr4,ericvergnaud/antlr4,parrt/antlr4,parrt/antlr4,parrt/antlr4,antlr/antlr4,ericvergnaud/antlr4,parrt/antlr4,ericvergnaud/antlr4,parrt/antlr4,ericvergnaud/antlr4,antlr/antlr4,antlr/antlr4,antlr/antlr4,parrt/antlr4,antlr/antlr4,antlr/antlr4,ericvergnaud/antlr4,parrt/antlr4,ericvergnaud/antlr4,ericvergnaud/antlr4,antlr/antlr4,ericvergnaud/antlr4,parrt/antlr4,parrt/antlr4