AsmParser: Move MDField details to source file, NFC

Move all the types of `MDField` to an anonymous namespace in the source
file.  This also eliminates the duplication of `ParseMDField()`
declarations in the header for each new field type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228211 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed