[corlib] TypeSpec.Parse for non-assembly qualified names.

[corlib] TypeSpec.Parse for non-assembly qualified names.

Previously for types like "Foo[P,Q]" the recursive call of the parser
would incorrectly consume the delimiter after a type argument (ie the
',' or ']') which would leave the caller potentially past the end of the
string which would signal a failed parse.  New behavior is to peek at
the delimiter but leave it up to the caller to consume.

Also `Type.GetType(name, null, null)` should behave like
`Type.GetType(name)` (previously it dropped generic type arguments).