[frontend] Don't destroy the compiler instance object inside performCompile()

[akyrtzi]

This permits continuing to use the diagnostic engine object from the instance.

[akyrtzi]

@swift-ci smoke test

[slavapestov]

It would be cleaner to construct the diagnostic engine outside of the compiler instance and pass it in to the instance. Then you can still destroy the entire compiler instance.

[swiftix]

Does it have any impact on the memory consumption? In general, we tried to free the memory occupied by the compiler ASAP, to reduce the peak memory usage.

[akyrtzi]

A previous suggestion was to have CompilerInstance keep a shared pointer to a DiagnosticEngine object, and also have the SourceManager reference be a shared pointer since DiagnosticEngine references it, but @DougGregor was not a fan of this, since the DiagnosticEngine in general keeps state that is relevant to a compiler instance (like whether a fatal error occurred, etc.)

[akyrtzi]

@swiftix , the commit still frees the ASTContext and SILModule object, which should account for the vast majority of the memory consumption.

[akyrtzi]

@swift-ci smoke test

[jrose-apple]

cc @mxswd

[mxswd]

I will try to get a build with this change to evaluate the difference in peak memory usage before merging.

[DougGregor]

LGTM, but let's see what Maxwell finds

[mxswd]

I've double checked and it looks great to me! Thanks!