This is a proposed solution to D30371. libfuzzer and compiler-rt in general often need to pass lists of files as inputs to the tools, frequently involving wildcard expressions, and this is not easy to do portably given the difference in environment between Windows and non-Windows platforms. Usually it involves some hacky call to xargs, but then when you mix find into the equation you run into trouble with Windows tools.
This patch adds a new lit substitution, %[[expr]] which will be treated as a glob expression and replaced in the input. Note that all glob expressions are expanded before doing any other substitutions, and because of this you can use traditional substitutions as part of a glob expression, which allows you to root your paths %S, %T, etc.
A simple test is added to demonstrate the use of the syntax.