In commit v3.4.0-rc2~1^2 (cmIfCommand: Issue CMP0054 warning with
appropriate context, 2015-10-20) we added construction of a
cmListFileContext with conversion of the calling file path.  This code
path runs on every condition (e.g. `if()`) and so the path conversion
has a noticeable performance cost.  Fortunately the only use of this
context is for insertion into cmMakefile::CMP0054ReportedIds so we do
not need to convert the path.  Simply drop the conversion.