C bindings for libLLVMCore.a and libLLVMBitWriter.a.

- The naming prefix is LLVM.
- All types are represented using opaque references.
- Functions are not named LLVM{Type}{Method}; the names became
  unreadable goop. Instead, they are named LLVM{ImperativeSentence}.
- Where an attribute only appears once in the class hierarchy (e.g.,
  linkage only applies to values; parameter types only apply to
  function types), the class is omitted from identifiers for
  brevity. Tastes like methods.
- Strings are C strings or string/length tuples on a case-by-case
  basis.
- APIs which give the caller ownership of an object are not mapped
  (removeFromParent, certain constructor overloads). This keeps
  keep memory management as simple as possible.

For each library with bindings:

  llvm-c/<LIB>.h       - Declares the bindings.
  lib/<LIB>/<LIB>.cpp  - Implements the bindings.

So just link with the library of your choice and use the C header
instead of the C++ one.

llvm-svn: 42077