Revert "Ninja: Use full path for all source files"

This reverts commit v3.7.0-rc1~275^2 (Ninja: Use full path for all
source files, 2016-08-05).  Unfortunately using absolute paths can
cause incorrect rebuilds due to ninja limitations.  The ninja
manual [1] explains:

> ... using absolute paths, your depfile may result in a mixture of
> relative and absolute paths. Paths used by other build rules need
> to match exactly.

Passing an absolute path to a source file to the compiler while using a
relative path in the ninja build manifest can cause such mixture and
lead to incorrect rebuilds.  Simply revert the change for now.

Note that there was a follow-up to the original change in commit
v3.7.0-rc2~10^2 (Ninja: Fix RC language depfile generation with
cmcldeps, 2016-10-13).  We don't need to revert that because that
change made the relevant code cleverly adapt to whatever variable
we use to reference the source file.

[1] https://ninja-build.org/manual.html#_deps

Fixes: #16675
Issue: #13894

Source/cmNinjaTargetGenerator.cxx

@@ -377,7 +377,7 @@ void cmNinjaTargetGenerator::WriteCompileRule(const std::string& lang)
   vars.RuleLauncher = "RULE_LAUNCH_COMPILE";
   vars.CMTarget = this->GetGeneratorTarget();
   vars.Language = lang.c_str();
-  vars.Source = "$IN_ABS";
+  vars.Source = "$in";
   vars.Object = "$out";
   vars.Defines = "$DEFINES";
   vars.Includes = "$INCLUDES";
@@ -729,7 +729,8 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatement(
   cmSourceFile const* source, bool writeOrderDependsTargetForTarget)
 {
   std::string const language = source->GetLanguage();
-  std::string const sourceFileName = this->GetSourceFilePath(source);
+  std::string const sourceFileName =
+    language == "RC" ? source->GetFullPath() : this->GetSourceFilePath(source);
   std::string const objectDir =
     this->ConvertToNinjaPath(this->GeneratorTarget->GetSupportDirectory());
   std::string const objectFileName =
@@ -738,8 +739,6 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatement(
     cmSystemTools::GetFilenamePath(objectFileName);
 
   cmNinjaVars vars;
-  vars["IN_ABS"] = this->GetLocalGenerator()->ConvertToOutputFormat(
-    source->GetFullPath(), cmOutputConverter::SHELL);
   vars["FLAGS"] = this->ComputeFlagsForObject(source, language);
   vars["DEFINES"] = this->ComputeDefines(source, language);
   vars["INCLUDES"] = this->GetIncludes(language);