do not use compiler.getSourceManager().getFilename()

This is a continuation of ff002524c1.
When compiling with icecream, its -frewrite-includes merges
all #include's into one .cxx file and marks them with with line
markers. But SourceManager::getFilename() doesn't take those
into account and so it reports all of those as <stdin>.
So use getFileNameOfSpellingLoc(), which explicitly handles this
case.
And we should probably never ever use SourceManager::getFilename().

Change-Id: Ia194c2e041578e1e199aee2df2f885922ef7e31a
Reviewed-on: https://gerrit.libreoffice.org/80326
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

compilerplugins/clang/plugin.cxx

@@ -621,8 +621,7 @@ bool RewritePlugin::wouldRewriteWorkdir(SourceLocation loc)
         return false;
     }
     return
-        compiler.getSourceManager().getFilename(
-            compiler.getSourceManager().getSpellingLoc(loc))
+        getFileNameOfSpellingLoc(compiler.getSourceManager().getSpellingLoc(loc))
         .startswith(WORKDIR "/");
 }