Clean "./" from __FILE__ expansion.

[ppluzhnikov]

Summary

This is alternative to https://reviews.llvm.org/D121733
and helps with Clang header modules in which FILE
may expand to "./foo.h" or "foo.h" depending on whether the file was
included directly or not.

Only do this when UseTargetPathSeparator is true, as we are already
changing the path in that case.

[ppluzhnikov]

Also fix a buglet in remove_dots().

[ppluzhnikov]

Rebase to ToT.

[ppluzhnikov]

Alan,

Thanks for the approval.
Could you push this patch -- I don't have commit rights.

[ayzhao]

In D126396#3554701, @ppluzhnikov wrote:

Alan,

Thanks for the approval.
Could you push this patch -- I don't have commit rights.

Done

[MaskRay]

Is it feasible to add a test?

[ppluzhnikov]

In D126396#3556321, @MaskRay wrote:

Is it feasible to add a test?

Sure. I extended the existing tests added in https://reviews.llvm.org/D122766 to cover this.
https://reviews.llvm.org/D127009

clang/lib/Lex/PPMacroExpansion.cpp

 }
 
 void Preprocessor::processPathForFileMacro(SmallVectorImpl<char> &Path,
                                            const LangOptions &LangOpts,
                                            const TargetInfo &TI) {
   LangOpts.remapPathPrefix(Path);
   if (LangOpts.UseTargetPathSeparator) {
     if (TI.getTriple().isOSWindows())
-      llvm::sys::path::make_preferred(
-          Path, llvm::sys::path::Style::windows_backslash);
+      llvm::sys::path::remove_dots(Path, false,
+                                   llvm::sys::path::Style::windows_backslash);
     else
-      llvm::sys::path::make_preferred(Path, llvm::sys::path::Style::posix);
+      llvm::sys::path::remove_dots(Path, false, llvm::sys::path::Style::posix);
   }
 }