[AsmPrinterDwarf] Add support for .cfi_restore directive

As of today we only use .cfi_offset to specify the offset of a CSR, but
we never use .cfi_restore when the CSR is restored.

If we want to perform a more advanced type of shrink-wrapping, we need
to use .cfi_restore in order to switch the CFI state between blocks.

This patch only aims at adding support for the directive.

Differential Revision: https://reviews.llvm.org/D36114

llvm-svn: 317199
diff --git a/llvm/lib/CodeGen/MIRParser/MILexer.h b/llvm/lib/CodeGen/MIRParser/MILexer.h
index 886916b..6894fe8 100644
--- a/llvm/lib/CodeGen/MIRParser/MILexer.h
+++ b/llvm/lib/CodeGen/MIRParser/MILexer.h
@@ -68,6 +68,7 @@
     kw_cfi_def_cfa_register,
     kw_cfi_def_cfa_offset,
     kw_cfi_def_cfa,
+    kw_cfi_restore,
     kw_blockaddress,
     kw_intrinsic,
     kw_target_index,