Requestify ExportedSourceFile parsing
Avoid parsing the syntax tree up-front, and instead
only parse it when required, which happens when either:
1. ASTGen parsing is enabled (currently disabled
by default)
2. Round trip checking is enabled for a primary
file (enabled by default in a debug build,
except when dep scanning or doing an IDE
operation)
3. We need to evaluate a macro in that file
This change therefore means that we now no longer
need to parse the syntax tree for secondary files
by default unless we specifically need to evaluate
a macro in them (e.g if we need to lookup a member
on a decl with an attached macro). And the same
for primaries in release builds.
rdar://109283847
