[clang-repl] Recommit "Land initial infrastructure for incremental parsing"

Original commit message:

  In http://lists.llvm.org/pipermail/llvm-dev/2020-July/143257.html we have
  mentioned our plans to make some of the incremental compilation facilities
  available in llvm mainline.

  This patch proposes a minimal version of a repl, clang-repl, which enables
  interpreter-like interaction for C++. For instance:

  ./bin/clang-repl
  clang-repl> int i = 42;
  clang-repl> extern "C" int printf(const char*,...);
  clang-repl> auto r1 = printf("i=%d\n", i);
  i=42
  clang-repl> quit

  The patch allows very limited functionality, for example, it crashes on invalid
  C++. The design of the proposed patch follows closely the design of cling. The
  idea is to gather feedback and gradually evolve both clang-repl and cling to
  what the community agrees upon.

  The IncrementalParser class is responsible for driving the clang parser and
  codegen and allows the compiler infrastructure to process more than one in...