Previously, libclang was run on each fragment (import "C") separately.
However, in regular Go it's possible for later fragments to refer to
types in earlier fragments so they must have been parsed as one.
This commit changes the behavior to run only one C parser invocation for
each Go file.
This commit adds tests for CGo preprocessing. There are various errors
that can be reported while preprocessing, and they should integrate well
with the compiler (including accurate source location tracking).
Also allow CGo preprocessing to continue after Clang encountered an
error, for a better view of what happened.
