SIL tools always operate as -parse-stdlib, even when working with modules.

r18796 added a check that the standard library is always available when
not working in -parse-stdlib or SIL mode, but sil-opt and sil-extract have
no need to load the stdlib if the module file they're working on does not
depend on it. Treat these tools as if they are always in a -parse-stdlib mode.

This fixes the failure in witness_tables.sil, which is due to us pulling in
witness tables from all loaded modules.

Swift SVN r18807

Author: jrose-apple

tools/sil-extract/SILExtract.cpp

@@ -85,6 +85,7 @@ int main(int argc, char **argv) {
   // Give the context the list of search paths to use for modules.
   Invocation.setImportSearchPaths(ImportPaths);
   Invocation.getClangImporterOptions().ModuleCachePath = ModuleCachePath;
+  Invocation.setParseStdlib();
 
   // Load the input file.
   std::unique_ptr<llvm::MemoryBuffer> InputFile;

tools/sil-opt/SILOpt.cpp

@@ -436,6 +436,7 @@ int main(int argc, char **argv) {
   // Set the module cache path. If not passed in we use the default swift module
   // cache.
   Invocation.getClangImporterOptions().ModuleCachePath = ModuleCachePath;
+  Invocation.setParseStdlib();
 
   // Load the input file.
   std::unique_ptr<llvm::MemoryBuffer> InputFile;