[gardening] Remove unreachable/unused/redundant code

* Make parameter naming in forward declaration match definition
* Remove unused argument to function persistAsync(…)
* Remove unused enum ShouldHalt
* Remove unused enum class IsProtocol
* Remove unused function dumpTypeSubstitutionMap()
* Remove unused function template getFirstPairElt(…)
* Remove unused method addConstantWordInWords(…)
* Remove unused method asExistentialTI()
* Remove unused method currentTrackedState()
* Remove unused method getNumBodyParameters()
* Remove unused method getSuccIndex()
* Remove unused method getTypeOfDeclReference(…)
* Remove unused method hasStructWithAtMostOneNonTrivialField(…)
* Remove unused method initForDirectValues()
* Remove unused method nextIfNot(…)
* Remove unused method overwriteLoweredValue(…)
* Remove unused method removeColumn(…)
* Remove unused methods HasSingleDecl() and GetFirstDecl()
* Remove unused methods overwriteLoweredExplosion(…) and setLoweredSingleValue(…)
* Remove unused methods requireRetainablePointerValue(…), getMethodSelfInstanceType(…) and isSelfArchetype(…)
* Remove unused methods setAsEmptyDirect(), setAsSingleDirectUnmanagedFragileValue(…), setAsIndirectAddress(…) and getDirectValues()
* Remove unused struct CachedMemberInfo
* Remove unused struct CallEdit
* Remove unused struct ErrorImportInfo
* Remove unused synonym ConformancePair
* Remove unused variable SemaInfo
* Remove unused variable localDeclNameNode
* Remove unused variables kindToken and kindLoc

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -2446,7 +2446,7 @@ ImmutableTextSnapshotRef SwiftEditorDocument::replaceText(
 }
 
 void SwiftEditorDocument::updateSemaInfo() {
-  if (auto SemaInfo = Impl.SemanticInfo) {
+  if (Impl.SemanticInfo) {
     Impl.SemanticInfo->processLatestSnapshotAsync(Impl.EditableBuffer);
   }
 }