Extend BasicBlock sections to allow specifying clusters of basic blocks

in the same section.

This allows specifying BasicBlock clusters like the following example:
!foo
!!0 1 2
!!4
This places basic blocks 0, 1, and 2 in one section in this order, and
places basic block #4 in a single section of its own.

Author: rlavaee

llvm/include/llvm/CodeGen/AsmPrinter.h

@@ -151,6 +151,10 @@ class AsmPrinter : public MachineFunctionPass {
   // function. This is used to calculate the size of the BB section.
   MCSymbol *CurrentSectionBeginSym = nullptr;
 
+  // The symbol used to represent the start of the current BB section of the
+  // function. This is used to calculate the size of the BB section.
+  MCSymbol *CurrentSectionBeginSym = nullptr;
+
   // The garbage collection metadata printer table.
   void *GCMetadataPrinters = nullptr; // Really a DenseMap.