Add a missing forward def of CheckerManager.  NFC.

This file doesn't include CheckerManager or forward declare it, so is sensitive to include order.

llvm-svn: 235209
diff --git a/clang/lib/StaticAnalyzer/Checkers/InterCheckerAPI.h b/clang/lib/StaticAnalyzer/Checkers/InterCheckerAPI.h
index b7549fd..d38d63c 100644
--- a/clang/lib/StaticAnalyzer/Checkers/InterCheckerAPI.h
+++ b/clang/lib/StaticAnalyzer/Checkers/InterCheckerAPI.h
@@ -13,6 +13,8 @@
 #ifndef LLVM_CLANG_LIB_STATICANALYZER_CHECKERS_INTERCHECKERAPI_H
 #define LLVM_CLANG_LIB_STATICANALYZER_CHECKERS_INTERCHECKERAPI_H
 namespace clang {
+class CheckerManager;
+
 namespace ento {
 
 /// Register the checker which evaluates CString API calls.