[libclang] Introduce a new indexing mode where we skip function bodies

that were already parsed in the same "indexing session".

An indexing session is defined as using the same CXIndexAction object
for multiple clang_indexSourceFile calls.
Passing CXIndexOpt_SkipParsedBodiesInSession as an indexing option will
enable the mode where we try to skip bodies that were already parsed in
another translation unit.

If a function's body was skipped, the "flags" field in the CXIdxDeclInfo
structure will have "CXIdxDeclFlag_Skipped" bit was set.

llvm-svn: 169539