[MC] Split the layout part of MCAssembler::finish() into its own method. NFC.

Split a MCAssembler::layout() method out of MCAssembler::finish(). This allows
running the MCSections layout separately from the streaming of the output
file. This way if a client wants to use MC to generate section contents, but
emit something different than the standard relocatable object files it is
possible (llvm-dsymutil is such a client).

llvm-svn: 246008
diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
index 09a27fe..3f419b5 100644
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -852,14 +852,11 @@
   return std::make_pair(FixedValue, IsPCRel);
 }
 
-void MCAssembler::Finish() {
+void MCAssembler::layout(MCAsmLayout &Layout) {
   DEBUG_WITH_TYPE("mc-dump", {
       llvm::errs() << "assembler backend - pre-layout\n--\n";
       dump(); });
 
-  // Create the layout object.
-  MCAsmLayout Layout(*this);
-
   // Create dummy fragments and assign section ordinals.
   unsigned SectionIndex = 0;
   for (MCSection &Sec : *this) {
@@ -896,8 +893,6 @@
       llvm::errs() << "assembler backend - final-layout\n--\n";
       dump(); });
 
-  uint64_t StartOffset = OS.tell();
-
   // Allow the object writer a chance to perform post-layout binding (for
   // example, to set the index fields in the symbol data).
   getWriter().executePostLayoutBinding(*this, Layout);
@@ -931,6 +926,14 @@
       }
     }
   }
+}
+
+void MCAssembler::Finish() {
+  // Create the layout object.
+  MCAsmLayout Layout(*this);
+  layout(Layout);
+
+  uint64_t StartOffset = OS.tell();
 
   // Write the object file.
   getWriter().writeObject(*this, Layout);