[Strings] Add a string lowering pass using magic imports (#6497)

The latest idea for efficient string constants is to encode the constants in
the import names of their globals and implement fast paths in the engines for
materializing those constants at instantiation time without needing to parse
anything in JS. This strategy only works for valid strings (i.e. strings without
unpaired surrogates) because only valid strings can be used as import names in
the WebAssembly syntax.

Add a new configuration of the StringLowering pass that encodes valid string
contents in import names, falling back to the JSON custom section approach for
invalid strings.

To test this chang, update the printer to escape import and export names
properly and update the legacy parser to parse escapes in import and export
names properly. As a drive-by, remove the incorrect check in the parser that the
import module and base names are non-empty.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/passes/passes.h b/src/passes/passes.h
index 1b1ca99c6..23a9ea70b 100644
--- a/src/passes/passes.h
+++ b/src/passes/passes.h
@@ -156,6 +156,7 @@ Pass* createSimplifyLocalsNoTeeNoStructurePass();
 Pass* createStackCheckPass();
 Pass* createStringGatheringPass();
 Pass* createStringLoweringPass();
+Pass* createStringLoweringMagicImportPass();
 Pass* createStripDebugPass();
 Pass* createStripDWARFPass();
 Pass* createStripProducersPass();