DebugInfo: Add MDLocalScope, a legal scope for locals

Add a subclass of `MDScope` to explicitly categorize the legal scopes
for locals -- in particular, scopes that are legal for `MDLocation`,
`MDLexicalBlockBase`, and `MDLocalVariable`.  This provides a convenient
`isa<>` target for the verifier, and eventually I'll be changing the
above classes' `getScope()` to specifically return it.  Currently, its
subclasses are `MDSubprogram`, `MDLexicalBlock`, and
`MDLexicalBlockFile`.

I've gone with `MDLocalScope` for now -- a little ambiguous since it's a
scope *for* locals, not a scope that's local -- but I'm open to more
descriptive names if someone can think of something better.  Regardless,
the code docs should make it clear enough.

llvm-svn: 233092