[Formatters][NFCI] Replace 'is_regex' arguments with an enum.

Modify `SBTypeNameSpecifier` and `lldb_private::TypeMatcher` so they
have an enum value for the type of matching to perform instead of an
`m_is_regex` boolean value.

This change paves the way for introducing formatter matching based on
the result of a python callback in addition to the existing name-based
matching. See the RFC thread at
https://discourse.llvm.org/t/rfc-python-callback-for-data-formatters-type-matching/64204
for more details.

Differential Revision: https://reviews.llvm.org/D133240