Add a missing forward def of CheckerManager. NFC.

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@235209 91177308-0d34-0410-b5e6-96231b3b80d8

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.