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

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).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@110210 91177308-0d34-0410-b5e6-96231b3b80d8