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.

clang/lib/Lex/PPMacroExpansion.cpp

if (MI->isWarnIfUnused() && !MI->isUsed())
     WarnUnusedMacroLocs.erase(MI->getDefinitionLoc());
   MI->setIsUsed(true);
 }
 
 void Preprocessor::processPathForFileMacro(SmallVectorImpl<char> &Path,
                                            const LangOptions &LangOpts,
                                            const TargetInfo &TI) {
   LangOpts.remapPathPrefix(Path);
+  llvm::sys::path::remove_dots(Path);
   if (LangOpts.UseTargetPathSeparator) {
     if (TI.getTriple().isOSWindows())
       llvm::sys::path::make_preferred(
           Path, llvm::sys::path::Style::windows_backslash);
     else
       llvm::sys::path::make_preferred(Path, llvm::sys::path::Style::posix);
   }
 }

clang/test/Modules/filename.cpp

 // RUN: cd %S
 // RUN: %clang_cc1 -I. -fmodule-name=A  -fmodule-map-file=%S/Inputs/filename/module.map %s -E | FileCheck %s
 
 #include "Inputs/filename/a.h"
 
 // Make sure that headers that are referenced by module maps have __FILE__
 // reflect the include path they were found with.
-// CHECK: const char *p = "./Inputs/filename/a.h"
+// CHECK: const char *p = "Inputs/filename/a.h"