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.

llvm-svn: 230111
diff --git a/llvm/utils/vim/llvm.vim b/llvm/utils/vim/llvm.vim
index 6b4b8bb..913d0f5 100644
--- a/llvm/utils/vim/llvm.vim
+++ b/llvm/utils/vim/llvm.vim
@@ -82,6 +82,7 @@
 syn match   llvmConstant /\<DW_OP_[a-zA-Z0-9_]\+\>/
 syn match   llvmConstant /\<DW_LANG_[a-zA-Z0-9_]\+\>/
 syn match   llvmConstant /\<DW_VIRTUALITY_[a-z_]\+\>/
+syn match   llvmConstant /\<DIFlag[A-Za-z]\+\>/
 
 " Syntax-highlight dejagnu test commands.
 syn match  llvmSpecialComment /;\s*RUN:.*$/