[libcxx] [test] Don't rerun supportsVerify for each individual test

We can't just memoize _supportsVerify in place in format.py, as it
previously was executed in each of the individual processes.

Instead use hasCompileFlag() and add a feature flag for it instead,
which can be used both by tests (that already have such a flag,
locally for one set of tests) and for the testing framework itself.

By using hasCompileFlag(), this also implicitly fixes two other issues:
Previously, _supportsVerify called subprocess.call() directly, which can
interpret command line quoting differently than lit.TestRunner.

(In particular, TestRunner handles arguments quoted by a single quote,
while launching Windows processes with subprocess.call() only supports
double quotes. This allows using shlex.quote(), which uses single quotes,
everywhere - as all commands now go through TestRunner. This should make
41d79093 redundant.)

Secondly, the old _supportsVerify method didn't include %{flags) or
%{compile_flags}.

Differential Revision: https://reviews.llvm.org/D116010