Make ID Parsing More Flexible

Add a mode control to value and ID parsers.  The two modes are:

- Parse a value.  Expect the parsed ID to map to an existing object.

- Parse a name.  Expect the parsed ID to not map to any existing object.

The first is used when parsing an identifier to be looked up, for
example a record field or template argument.  The second is used for
parsing declarations.  Paste functionality implies that declarations
can contain arbitrary expressions so we need to be able to call into
the general value parser to parse declarations with paste operators.
So we need a way to parse a value-like thing without expecting that
the result will map to some existing object.  This parse mode provides
that.

llvm-svn: 142519