Rename ninja variables with Clang in the name

Rename the ninja variables that have an extraneous Clang in the name,
since there are no longer any non-Clang variables.

Bug: 68947919
Test: no change to command lines used to build mmma bionic
Change-Id: I081d5e1d423a66d55faece6cb1d265cd5e504d5f
diff --git a/cc/config/bp2build.go b/cc/config/bp2build.go
index 19571f1..e7e94a8 100644
--- a/cc/config/bp2build.go
+++ b/cc/config/bp2build.go
@@ -118,7 +118,7 @@
 // quite complex to track depth-first interpolations. It's also unlikely the
 // interpolation stacks are deep (n > 1).
 func expandVar(toExpand string, stringScope exportedStringVariables, stringListScope exportedStringListVariables) []string {
-	// e.g. "${ClangExternalCflags}"
+	// e.g. "${ExternalCflags}"
 	r := regexp.MustCompile(`\${([a-zA-Z0-9_]+)}`)
 
 	// Internal recursive function.