CMake configuration is broken on case sensitive filesystems

[lorentey]

Swift Collections 1.1.0 has a typo in its CMake configuration, which prevents it from working on case sensitive filesystems.

From swiftlang/swift#73654:

CMake Error at Sources/OrderedCollections/CMakeLists.txt:10 (add_library):
  Cannot find source file:

    Utilities/_UnsafeBitSet.swift

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm .h
  .hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90 .f95 .f03 .hip .ispc


CMake Error at Sources/OrderedCollections/CMakeLists.txt:10 (add_library):
  No SOURCES given to target: OrderedCollections

[lorentey]

Fixed in #377