Cut off parsing early during code completion

These calls to ConsumeCodeCompletionToken() caused parsing to continue
needlessly when an immediate cutOffParsing() would do.

Document the function to clarify its correct usage.
Remove unused IncrProcessing parameter from Preprocessor ctor

Preprocessor::enableIncrementalProcessing() provides a consistent interface to
enable the feature.
Factor TargetInfo pointer/DelayInitialization bool pair out of Preprocessor ctor

The Preprocessor::Initialize() function already offers a clear interface to
achieve this, further reducing the confusing number of states a newly
constructed preprocessor can have.
Reformat code following Preprocessor constructor updates

Landing this separately to make the previous commits easy to follow at home.
[PECOFF] Support =internalName syntax in .def file.

Export definitions in a module definition file is as follows:

  exportedname[=internalname] [@ordinal [NONAME]] [PRIVATE] [DATA]

Previously we did not support =internalname, so users couldn't export
symbols from a DLL with a different name.