AsmParser/Writer: Handle symbolic constants in DI 'flags:'

Parse (and write) symbolic constants in debug info `flags:` fields.
This prevents a readability (and CHECK-ability) regression with the new
debug info hierarchy.

Old (well, current) assembly, with pretty-printing:

    !{!"...\\0016387", ...} ; ... [public] [rvalue reference]

Flags field without this change:

   !MDDerivedType(flags: 16387, ...)

Flags field with this change:

   !MDDerivedType(flags: DIFlagPublic | DIFlagRValueReference, ...)

As discussed in the review thread, this isn't a final state.  Most of
these flags correspond to `DW_AT_` symbolic constants, and we might
eventually want to support arbitrary attributes in some form.  However,
as it stands now, some of the flags correspond to other concepts (like
`FlagStaticMember`); until things are refactored this is the simplest
way to move forward without regressing assembly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230111 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AsmParser/LLToken.h

@@ -29,6 +29,7 @@ namespace lltok {
     less, greater,     // <  >
     lparen, rparen,    // (  )
     exclaim,           // !
+    bar,               // |
 
     kw_x,
     kw_true,    kw_false,
@@ -203,6 +204,7 @@ namespace lltok {
     DwarfVirtuality,   // DW_VIRTUALITY_foo
     DwarfLang,         // DW_LANG_foo
     DwarfOp,           // DW_OP_foo
+    DIFlag,            // DIFlagFoo
 
     // Type valued tokens (TyVal).
     Type,