clang-plugin: Move type lookups out into a new TypeManager

This implements caching of the looked up types (which should improve
performance) and also centralises the implementation so it can be used
by other checkers.
clang-plugin: Port GVariant checker to use TypeManager

This eliminates the horrible FORCE_* flag hacks which were needed for
comparing non-basic types.
clang-plugin: Simplify GVariant checker error message code

This standardises all error messages on using the Clang-internal
QualType formatting functions, improving consistency.
clang-plugin: Fix error message formatting in GVariant checker

The Clang QualType formatting adds quotation marks, but our error
messages already contained them. Loath as I am to revert to using ASCII
quotation marks, the formatting provided by QualType is quite good
(since it adds aka information), so drop the old Tartan Unicode
quotation marks.
website: Improve usage instruction formatting and wording

Hopefully this makes things a little clearer as to how to use Tartan.
scripts: Improve error handling in tartan script

Actually kill the script if Clang or the plugin library cannot be found.
Emit an error if the script is called with no arguments (in this case
Clang would also emit an error if we called it, but the Clang error is
quite uninformative).
scripts: Remove an unnecessarily hardcoded string