[LLDB] Expose several methods in SBWatchpoint

This patch adds the following methods:

* `GetType()`
* `GetWatchValueKind()`
* `GetWatchSpec()`
* `IsWatchingReads()`
* `IsWatchingWrites()`

These mostly expose methods that `lldb_private::Watchpoint` already
had. Tests are included that exercise these new methods.

The motivation for exposing these are as follows:

* `GetType()` - With this information and the address from a watchpoint
  it is now possible to construct an SBValue from an SBWatchpoint.
  Previously this wasn't possible. The included test case illustrates
  doing this.
* `GetWatchValueKind()` - This allows the caller to determine whether the
  watchpoint is a variable watchpoint or an expression watchpoint. A new
  enum (`WatchpointValueKind`) has been introduced to represent the
  return values. Unfortunately the name `WatchpointKind` was already
  taken.
* `GetWatchSpec()` - This allows (at least for variable watchpoints)
  to use a sensible name for SBValues created from an SBWatchpoint.
* `IsWatchingReads()` - This allow checking if a watchpoint is
  monitoring read accesses.
* `IsWatchingWRites()` - This allow checking if a watchpoint is
  monitoring write accesses.

rdar://105606978

Reviewers: jingham, mib, bulbazord, jasonmolenda, JDevlieghere

Differential Revision: https://reviews.llvm.org/D144937