DebugInfo: Improve reuse of file table entries in asm debug info

The previous deduping strategy was woefully inadequate - it only
considered the most recent file used and avoided emitting a duplicate in
that case - never considering the a/b/a scenario.

It was also lacking when it came to directory paths as the previous
filename would never match the current if the filename had been split
into file and directory components.

This change builds caching functionality into the line table at the
lowest level in an optional form (a file number of 0 indicates that one
should be chosen and returned) and will eventually be reused by the
normal source level debugging DWARF emission.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204027 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCNullStreamer.cpp

@@ -85,10 +85,10 @@ namespace {
                            unsigned char Value = 0) override { return false; }
 
     void EmitFileDirective(StringRef Filename) override {}
-    bool EmitDwarfFileDirective(unsigned FileNo, StringRef Directory,
-                                StringRef Filename,
-                                unsigned CUID = 0) override {
-      return false;
+    unsigned EmitDwarfFileDirective(unsigned FileNo, StringRef Directory,
+                                    StringRef Filename,
+                                    unsigned CUID = 0) override {
+      return 0;
     }
     void EmitDwarfLocDirective(unsigned FileNo, unsigned Line,
                                unsigned Column, unsigned Flags,