if,while: Clarify condition backtrace construction

Evaluation of the `elseif`, `else`, and `while` commands takes place
during function blocker evaluation before any actual command execution
takes place.  Therefore they do not have an entry in the backtrace
stack.  Each of their implementations needs to construct an extra
backtrace entry to use in error messages and such.  Each of them used a
slightly different approach due to evolution over time.  Clean up their
construction of the extra backtrace entry and use a named variable to
contain it for clarity.

Source/cmIfCommand.cxx

@@ -75,10 +75,12 @@ bool cmIfFunctionBlocker::Replay(std::vector<cmListFileFunction> functions,
     if (scopeDepth == 0 && func.Name.Lower == "else") {
 
       if (this->ElseSeen) {
-        cmListFileBacktrace bt = mf.GetBacktrace(func);
+        cmListFileBacktrace elseBT = mf.GetBacktrace().Push(
+          cmListFileContext{ func.Name.Original,
+                             this->GetStartingContext().FilePath, func.Line });
         mf.GetCMakeInstance()->IssueMessage(
           MessageType::FATAL_ERROR,
-          "A duplicate ELSE command was found inside an IF block.", bt);
+          "A duplicate ELSE command was found inside an IF block.", elseBT);
         cmSystemTools::SetFatalErrorOccured();
         return true;
       }
@@ -93,11 +95,12 @@ bool cmIfFunctionBlocker::Replay(std::vector<cmListFileFunction> functions,
         mf.PrintCommandTrace(func);
       }
     } else if (scopeDepth == 0 && func.Name.Lower == "elseif") {
+      cmListFileBacktrace elseifBT = mf.GetBacktrace().Push(cmListFileContext{
+        func.Name.Original, this->GetStartingContext().FilePath, func.Line });
       if (this->ElseSeen) {
-        cmListFileBacktrace bt = mf.GetBacktrace(func);
         mf.GetCMakeInstance()->IssueMessage(
           MessageType::FATAL_ERROR,
-          "An ELSEIF command was found after an ELSE command.", bt);
+          "An ELSEIF command was found after an ELSE command.", elseifBT);
         cmSystemTools::SetFatalErrorOccured();
         return true;
       }
@@ -122,7 +125,7 @@ bool cmIfFunctionBlocker::Replay(std::vector<cmListFileFunction> functions,
             func, this->GetStartingContext().FilePath);
 
         cmConditionEvaluator conditionEvaluator(mf, conditionContext,
-                                                mf.GetBacktrace(func));
+                                                elseifBT);
 
         bool isTrue =
           conditionEvaluator.IsTrue(expandedArguments, errorString, messType);
@@ -130,8 +133,7 @@ bool cmIfFunctionBlocker::Replay(std::vector<cmListFileFunction> functions,
         if (!errorString.empty()) {
           std::string err =
             cmStrCat(cmIfCommandError(expandedArguments), errorString);
-          cmListFileBacktrace bt = mf.GetBacktrace(func);
-          mf.GetCMakeInstance()->IssueMessage(messType, err, bt);
+          mf.GetCMakeInstance()->IssueMessage(messType, err, elseifBT);
           if (messType == MessageType::FATAL_ERROR) {
             cmSystemTools::SetFatalErrorOccured();
             return true;

Source/cmWhileCommand.cxx

@@ -17,6 +17,7 @@
 #include "cmMakefile.h"
 #include "cmMessageType.h"
 #include "cmSystemTools.h"
+#include "cmake.h"
 
 class cmWhileFunctionBlocker : public cmFunctionBlocker
 {
@@ -66,14 +67,10 @@ bool cmWhileFunctionBlocker::Replay(std::vector<cmListFileFunction> functions,
   mf.ExpandArguments(this->Args, expandedArguments);
   MessageType messageType;
 
-  cmListFileContext execContext = this->GetStartingContext();
-
-  cmCommandContext commandContext;
-  commandContext.Line = execContext.Line;
-  commandContext.Name = execContext.Name;
-
+  cmListFileBacktrace whileBT =
+    mf.GetBacktrace().Push(this->GetStartingContext());
   cmConditionEvaluator conditionEvaluator(mf, this->GetStartingContext(),
-                                          mf.GetBacktrace(commandContext));
+                                          whileBT);
 
   bool isTrue =
     conditionEvaluator.IsTrue(expandedArguments, errorString, messageType);
@@ -90,7 +87,7 @@ bool cmWhileFunctionBlocker::Replay(std::vector<cmListFileFunction> functions,
       err += "(";
       err += errorString;
       err += ").";
-      mf.IssueMessage(messageType, err);
+      mf.GetCMakeInstance()->IssueMessage(messageType, err, whileBT);
       if (messageType == MessageType::FATAL_ERROR) {
         cmSystemTools::SetFatalErrorOccured();
         return true;