Frontend: Introduce -emit-api-descriptor flag.

An "API descriptor" file is JSON describing the externally accessible symbols
of a module and metadata associated with those symbols like availability and
SPI status. This output was previously only generated by the
`swift-api-extract` alias of `swift-frontend`, which is desgined to take an
already built module as input. Post-processing a built module to extract this
information is inefficient because the module and the module's dependencies
need to be deserialized in order to visit the entire AST. We can generate this
output more efficiently as a supplementary output of the -emit-module job that
originally produced the module (since the AST is already available in-memory).
The -emit-api-descriptor flag can be used to request this output.

This change lays the groundwork by introducing frontend flags. Follow up
changes are needed to make API descriptor emission during -emit-module
functional.

Part of rdar://110916764.

lib/Frontend/FrontendInputsAndOutputs.cpp

@@ -511,6 +511,12 @@ bool FrontendInputsAndOutputs::hasABIDescriptorOutputPath() const {
         return outs.ABIDescriptorOutputPath;
       });
 }
+bool FrontendInputsAndOutputs::hasAPIDescriptorOutputPath() const {
+  return hasSupplementaryOutputPath(
+      [](const SupplementaryOutputPaths &outs) -> const std::string & {
+        return outs.APIDescriptorOutputPath;
+      });
+}
 bool FrontendInputsAndOutputs::hasConstValuesOutputPath() const {
   return hasSupplementaryOutputPath(
       [](const SupplementaryOutputPaths &outs) -> const std::string & {