[libc++] Allow specifying conditional compile flags dependent on basic Lit features

This patch adds support for passing basic Lit features to the
ADDITIONAL_COMPILE_FLAGS keyword by enclosing them in parentheses.
This is done to support https://llvm.org/D131836.

In the future, we should instead add proper support for conditional
keywords in Lit, so that we can evaluate arbitrary Lit boolean
expressions such as `ADDITIONAL_COMPILE_FLAGS(x && !y): -flag`.

Note that I can see this being exceptionally useful when combined
with RUN commands, which would allow using different commands on
different systems. For example:

     RUN(!buildhost=windows): something
     RUN(buildhost=windows): something-else

Differential Revision: https://reviews.llvm.org/D132575