MachO: Add linker-optimisation hint framework to MC.

Another part of the ARM64 backend (so tests will be following soon).
This is currently used by the linker to relax adrp/ldr pairs into nops
where possible, though could well be more broadly applicable.

llvm-svn: 205084
diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
index 5536087..724ca29 100644
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -355,6 +355,7 @@
   getBackend().reset();
   getEmitter().reset();
   getWriter().reset();
+  getLOHContainer().reset();
 }
 
 bool MCAssembler::isSymbolLinkerVisible(const MCSymbol &Symbol) const {