compilerplugins: try to make these work with icecream

There are some problems here, this should fix one of them: the
getFilename function returns "<stdin>" for spelling locations, because
the input to clang is sort of preprocessed via -frewrite-includes if
icecream is used and the file is built on a remote host (whereas it's
apparently not preprocessed if the file is compiled locally by icecream).

Using getPresumedLoc() uses the #line directives in the preprocessed
input, which avoids the problem but is more expensive, so try to use it
only when necessary.

The getFileEntry(getMainFileID())->getName() pattern will also result
in "<stdin>", but fortunately icecream passes -main-file-name,
which oddly enough isn't used by the SourceManager's spelling locations,
but is available separately via CodeGenOptions.

This builds everything successfully with clang version 6.0.0:
ICECC_PREFERRED_HOST=myremote make check gb_SUPPRESS_TESTS=t

Change-Id: Ic121511683e5302d7b9d85186c8b9c4a5443fa1b
Reviewed-on: https://gerrit.libreoffice.org/54993
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

compilerplugins/clang/nullptr.cxx

@@ -233,7 +233,7 @@ bool Nullptr::TraverseLinkageSpecDecl(LinkageSpecDecl * decl) {
 
 bool Nullptr::isInLokIncludeFile(SourceLocation spellingLocation) const {
     return loplugin::hasPathnamePrefix(
-        compiler.getSourceManager().getFilename(spellingLocation),
+        getFileNameOfSpellingLoc(spellingLocation),
         SRCDIR "/include/LibreOfficeKit/");
 }