Speed up code-completion by skipping function bodies.

When we are in code-completion mode, skip parsing of all function bodies except the one where the
code-completion point resides.

For big .cpp files like 'SemaExpr.cpp' the improvement makes a huge difference, in some cases cutting down
code-completion time -62% !

We don't get diagnostics for the bodies though, so modify the code-completion tests that check for errors.

See rdar://8814203.

llvm-svn: 122765