[AArch64] - Return address signing dwarf support

- Reapply changes intially introduced in r343089
- The archtecture info is no longer loaded whenever a DWARFContext is created
- The runtimes libraries (santiziers) make use of the dwarf context classes but
  do not intialise the target info
- The architecture of the object can be obtained without loading the target info
- Adding a method to the dwarf context to get this information and multiplex the
  string printing later on

Differential Revision: https://reviews.llvm.org/D55774

llvm-svn: 349472
diff --git a/llvm/lib/CodeGen/MIRParser/MILexer.h b/llvm/lib/CodeGen/MIRParser/MILexer.h
index a52f620..ceff7908 100644
--- a/llvm/lib/CodeGen/MIRParser/MILexer.h
+++ b/llvm/lib/CodeGen/MIRParser/MILexer.h
@@ -89,6 +89,7 @@
     kw_cfi_restore_state,
     kw_cfi_undefined,
     kw_cfi_window_save,
+    kw_cfi_aarch64_negate_ra_sign_state,
     kw_blockaddress,
     kw_intrinsic,
     kw_target_index,