Move File from SymbolBody to Symbol.

With this Symbol has the same size as before, but DefinedRegular goes
from 72 to 64 bytes.

I also find this a bit easier to read. There are fewer places
initializing File for example.

This has a small but measurable speed improvement on all tests (1%
max).

llvm-svn: 310142
diff --git a/lld/ELF/MapFile.cpp b/lld/ELF/MapFile.cpp
index ed0961c..1e466b7 100644
--- a/lld/ELF/MapFile.cpp
+++ b/lld/ELF/MapFile.cpp
@@ -53,7 +53,7 @@
   for (ObjFile<ELFT> *File : ObjFile<ELFT>::Instances)
     for (SymbolBody *B : File->getSymbols())
       if (auto *DR = dyn_cast<DefinedRegular>(B))
-        if (DR->File == File && !DR->isSection() && DR->Section &&
+        if (DR->getFile() == File && !DR->isSection() && DR->Section &&
             DR->Section->Live)
           V.push_back(DR);
   return V;