Treat .cxx files as c++

Fixes: 135749365
Fixes: 135667566
Test: m checkbuild
Change-Id: If47274812dfe781ebbb9f8a60a395650573727d5
diff --git a/cc/builder.go b/cc/builder.go
index 1e12361..ee40736 100644
--- a/cc/builder.go
+++ b/cc/builder.go
@@ -416,7 +416,7 @@
 			ccCmd = "clang"
 			moduleCflags = cflags
 			moduleToolingCflags = toolingCflags
-		case ".cpp", ".cc", ".mm":
+		case ".cpp", ".cc", ".cxx", ".mm":
 			ccCmd = "clang++"
 			moduleCflags = cppflags
 			moduleToolingCflags = toolingCppflags