IncrementalBuilder SyntaxTree cache

Incremental builder keeps in a cache at most one ParsedInput for each file it parses. This behavior can be toggled with useSyntaxTreeCache parameter.

Memory impact of this feature can be in range of tens of MB for larger solutions. This can be inspected in memory profilng tools by searching for ParsedInput instances. When partial checking is enabled, implementation files backed by signature will not be parsed or cached, as expected.