This patch adds a new `DoPrepareForExecution` API, which can be
implemented by the Clang and Swift language plugins. This also moves
`RunStaticInitializers` into `ExpressionParser::PrepareForExecution`, so
we call it consistently between language plugins.
This *should* be mostly NFC (the static initializers will still only run
after we finished parsing). We've been living on this patch downstream
for sometime now.
rdar://130267058
