Fix path separator issue on Windows

[ctetreau]

Summary

The following 3 tests fail on Windows:
Clang :: ClangScanDeps/header_stat_before_open.m
Clang :: ClangScanDeps/static-analyzer.c
Clang :: ClangScanDeps/vfsoverlay.cpp

The issue is that bad filenames are being constructed. This change uses
llvm::sys::path::append to concatenate file paths rather than string
append with a '/' thrown in the middle.

clang/lib/Lex/HeaderSearch.cpp

Optional<FileEntryRef> HeaderSearch::LookupFile(
   // This search is not done for <> headers.
   if (!Includers.empty() && !isAngled && !NoCurDirSearch) {
     SmallString<1024> TmpDir;
     bool First = true;
     for (const auto &IncluderAndDir : Includers) {
       const FileEntry *Includer = IncluderAndDir.first;
 
       // Concatenate the requested file onto the directory.
-      // FIXME: Portability.  Filename concatenation should be in sys::Path.
       TmpDir = IncluderAndDir.second->getName();
-      TmpDir.push_back('/');
-      TmpDir.append(Filename.begin(), Filename.end());
+      llvm::sys::path::append(TmpDir, Filename);
 
       // FIXME: We don't cache the result of getFileInfo across the call to
       // getFileAndSuggestModule, because it's a reference to an element of
       // a container that could be reallocated across this call.
       //
       // If we have no includer, that means we're processing a #include
       // from a module build. We should treat this as a system header if we're
       // building a [system] module.