[lldb] Fix detection of existing libcxx

The CMake variable LLDB_HAS_LIBCXX is passed to
`llvm_canonicalize_cmake_booleans`, which transforms TRUE/FALSE into
'1'/'0'. It also transforms undefined variables to '0'.

In particular, this means that the configuration script for LLDB API's
test always has _some_ value for the `has_libcxx` configuration:

```
config.has_libcxx = '@LLDB_HAS_LIBCXX@'
```

When deciding whether a libcxx exist, the testing scripts would only
check for the existence of `has_libcxx`, but not for its value. In other
words, because `if ('0')` is true in python we always think there is a
libcxx.

This was caught once D132940 was merged and most tests started to use
libcxx by default if `has_libcxx` is true.  Prior to that, no failures
were seen because only tests are marked with
`@add_test_categories(["libc++"])` would require a libcxx, and these
would be filtered out on builds without the libcxx target. Furthermore,
the MacOS bots always build libcxx.

We fix this by making `has_libcxx` a boolean (instead of a string) and
by checking its value in the test configuration.

Differential Revision: https://reviews.llvm.org/D133639