Don't use PATH_MAX.

This is a SmallVector anyway, and so the exact size doesn't matter.

clang\lib\Frontend\ModuleDependencyCollector.cpp(83) : error C2065: 'PATH_MAX' : undeclared identifier
clang\lib\Frontend\ModuleDependencyCollector.cpp(83) : error C2975: 'InternalLen' : invalid template argument for 'llvm::SmallString', expected compile-time constant expression
        llvm\include\llvm/ADT/SmallString.h(24) : see declaration of 'InternalLen'

llvm-svn: 265634