Extend -remap-file=from;to to permit mapping from a non-existent

file. This is accomplished by introducing the notion of a "virtual"
file into the file manager, which provides a FileEntry* for a named
file whose size and modification time are known but which may not
exist on disk.

Added a cute little test that remaps both a .c file and a .h file it
includes to alternative files.

llvm-svn: 90329

clang/lib/Frontend/InitPreprocessor.cpp

@@ -19,6 +19,7 @@
 #include "clang/Basic/FileManager.h"
 #include "clang/Basic/SourceManager.h"
 #include "llvm/ADT/StringExtras.h"
+#include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/System/Path.h"
@@ -502,11 +503,11 @@ static void InitializeFileRemapping(Diagnostic &Diags,
       continue;
     }
 
-    // Find the file that we're mapping from.
-    const FileEntry *FromFile = FileMgr.getFile(Remap->first);
+    // Create the file entry for the file that we're mapping from.
+    const FileEntry *FromFile = FileMgr.getVirtualFile(Remap->first,
+                                                       ToFile->getSize(),
+                                                       0);
     if (!FromFile) {
-      // FIXME: We could actually recover from this, by faking a
-      // FileEntry based on the "ToFile".
       Diags.Report(diag::err_fe_remap_missing_from_file)
         << Remap->first;
       continue;