Track property assignment the same way as property reassignment

[fsigalov]

When a property is reassigned, the log shows which file and line caused it, but for the first assignment of a property we just see the value it has, but not who assigned it, and why. This would really help in debugging into why a certain property has the wrong value 'from the start'.

VCRuntimeDLL = d:\depot\master2\packages\VisualCppTools.14.13.26131.1\lib\native\redist\debug_nonredist\x64\Microsoft.VC141.DebugCRT\vcruntime140d.dll
Property reassignment: $(VCRuntimeDLL)="d:\depot\master2\packages\VisualCppTools.14.13.26131.1\lib\native\redist\debug_nonredist\x64\Microsoft.VC141.DebugCRT\vcruntime140d.dll" (previous value: "d:\depot\master2\packages\VisualCppTools.14.13.26131.1\lib\native\redist\debug_nonredist\x64\Microsoft.VC141.DebugCRT\vcruntime141d.dll") at d:\depot\master2\MTBuild\MTBuild.ReferencedBits (126,5)