lit.py: Add new %{shared_output(LABEL)} substitution

This refers to a temporary path that can be shared across all tests,
identified by a particular label. This can be used for things like
caches.

At the moment, the character set for the LABEL is limited to C
identifier characters, plus '-', '+', '=', and '.'. This is the same
set of characters currently allowed in REQUIRES clause identifiers.

llvm-svn: 315697