Support code-completion for C++ inline methods and ObjC buffering methods.

Previously we would cut off the source file buffer at the code-completion
point; this impeded code-completion inside C++ inline methods and,
recently, with buffering ObjC methods.

Have the code-completion inserted into the source buffer so that it can
be buffered along with a method body. When we actually hit the code-completion
point the cut-off lexing or parsing.

Fixes rdar://10056932&8319466

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139086 91177308-0d34-0410-b5e6-96231b3b80d8