fix: LS not showing existing diagnostics on file open

[kyliau]

This commit fixes a bug whereby existing diagnostics in an external template
are not shown when the file is opened the first time. Diagnostics only show
up when subsequent edits are made to the file.
This is a regression after upgrading to TS 4.0

Fix #922

[ayazhafiz]

Why did this change? Is it to do with externalFiles?

[kyliau]

@ayazhafiz microsoft/TypeScript#37239 changed the implementation of openClientFile such that it no longer returns the configFileName when the opening of a file causes a new project to be created.
We actually saw this behavior earlier in #935. While the change is harmless on its own because we already handle subsequent project discovery with ProjectService.getDefaultProjectForScriptInfo(), I overlooked the fact that we also trigger diagnostics on file open. We should not return early when configFileName is absent. Instead we should work around it by looking at the containingProjects of the scriptInfo created by the call to openClientFiles().

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