AsmPrinter: Avoid EmitLabelDifference() in DwarfAccelTable

Mint a new function, `AsmPrinter::emitDwarfStringOffset()`, which takes
a `DwarfStringPoolEntryRef`.  When DWARF is relocatable across sections,
this defers to `emitSectionOffset()` and emits the `MCSymbol`;
otherwise, just emit the offset directly, without using any intermediate
symbols.

`EmitLabelDifference()` is already optimized to emit absolute label
differences cheaply when possible, so there aren't any major memory
savings here (853 MB down to 851 MB, or 0.2%).  However, it prepares for
making the `MCSymbol`s in the `DwarfStringPool` optional.

(I'm looking at `llc` memory usage on `verify-uselistorder.lto.opt.bc`;
see r236629 for details.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238119 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/AsmPrinter.h

@@ -19,6 +19,7 @@
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/DwarfStringPoolEntry.h"
 #include "llvm/IR/InlineAsm.h"
 #include "llvm/Support/DataTypes.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -422,6 +423,13 @@ public:
   /// or by emitting it as an offset from a label at the start of the section.
   void emitSectionOffset(const MCSymbol *Label) const;
 
+  /// Emit the 4-byte offset of a string from the start of its section.
+  ///
+  /// When possible, emit a DwarfStringPool section offset without any
+  /// relocations, and without using the symbol.  Otherwise, defers to \a
+  /// emitSectionOffset().
+  void emitDwarfStringOffset(DwarfStringPoolEntryRef S) const;
+
   /// Get the value for DW_AT_APPLE_isa. Zero if no isa encoding specified.
   virtual unsigned getISAEncoding() { return 0; }