Add code-completion support directly to ASTUnit, which performs code

completion within the translation unit using the same command-line
arguments for parsing the translation unit. Eventually, we'll reuse
the precompiled preamble to improve code-completion performance, and
this also gives us a place to cache results.

Expose this function via the new libclang function
clang_codeCompleteAt(), which performs the code completion within a
CXTranslationUnit. The completion occurs in-process
(clang_codeCompletion() runs code completion out-of-process).

llvm-svn: 110210