[Clang][unittests][NFC] Break up test in Callbacks.cpp

The Callbacks.cpp test was taking a long time to compile on some build bots
causing timeouts. This patch splits up that test into five separate cpp
files and a header file.

Reviewed By: gribozavr2

Differential Revision: https://reviews.llvm.org/D88886

clang/unittests/Tooling/CMakeLists.txt

@@ -20,7 +20,11 @@ add_clang_unittest(ToolingTests
   QualTypeNamesTest.cpp
   RangeSelectorTest.cpp
   RecursiveASTVisitorTests/Attr.cpp
-  RecursiveASTVisitorTests/Callbacks.cpp
+  RecursiveASTVisitorTests/CallbacksLeaf.cpp
+  RecursiveASTVisitorTests/CallbacksUnaryOperator.cpp
+  RecursiveASTVisitorTests/CallbacksBinaryOperator.cpp
+  RecursiveASTVisitorTests/CallbacksCompoundAssignOperator.cpp
+  RecursiveASTVisitorTests/CallbacksCallExpr.cpp
   RecursiveASTVisitorTests/Class.cpp
   RecursiveASTVisitorTests/Concept.cpp
   RecursiveASTVisitorTests/ConstructExpr.cpp