[NFC]: Rename a template parameter conflict with CALLBACK macro (microsoft#6759)

This imports upstream commit

llvm/llvm-project@54bff15
    
This fixes the following compiler error with clang 18.1.8 with mingw-w64
toolchain.
    
    CFG.h:916:22: error: expected a qualified name after 'typename'
      916 |   template <typename CALLBACK>
          |                      ^
    minwindef.h:90:18: note: expanded from macro 'CALLBACK'
       90 | #define CALLBACK __stdcall
          |                  ^

Author: Biswa96

tools/clang/include/clang/Analysis/CFG.h

@@ -913,8 +913,7 @@ class CFG {
   // Member templates useful for various batch operations over CFGs.
   //===--------------------------------------------------------------------===//
 
-  template <typename CALLBACK>
-  void VisitBlockStmts(CALLBACK& O) const {
+  template <typename Callback> void VisitBlockStmts(Callback &O) const {
     for (const_iterator I=begin(), E=end(); I != E; ++I)
       for (CFGBlock::const_iterator BI=(*I)->begin(), BE=(*I)->end();
            BI != BE; ++BI) {