[AutoDiff] Standardize on "differentiability parameter" terminology.

Change "differentiation parameter" to "differentiability parameter", matching
terminology in docs/DifferentiableProgramming.md.

Author: dan-zheng

lib/Parse/ParseDecl.cpp

@@ -877,7 +877,7 @@ bool Parser::parseDifferentiabilityParametersClause(
     SmallVectorImpl<ParsedAutoDiffParameter> &parameters, StringRef attrName,
     bool allowNamedParameters) {
   SyntaxParsingContext DiffParamsClauseContext(
-      SyntaxContext, SyntaxKind::DifferentiationParamsClause);
+      SyntaxContext, SyntaxKind::DifferentiabilityParamsClause);
   consumeToken(tok::identifier);
   if (!consumeIf(tok::colon)) {
     diagnose(Tok, diag::expected_colon_after_label, "wrt");
@@ -887,8 +887,8 @@ bool Parser::parseDifferentiabilityParametersClause(
   // Function that parses a parameter into `parameters`. Returns true if error
   // occurred.
   auto parseParam = [&](bool parseTrailingComma = true) -> bool {
-    SyntaxParsingContext DiffParamContext(
-        SyntaxContext, SyntaxKind::DifferentiationParam);
+    SyntaxParsingContext DiffParamContext(SyntaxContext,
+                                          SyntaxKind::DifferentiabilityParam);
     SourceLoc paramLoc;
     switch (Tok.getKind()) {
     case tok::identifier: {
@@ -932,8 +932,8 @@ bool Parser::parseDifferentiabilityParametersClause(
 
   // Parse opening '(' of the parameter list.
   if (Tok.is(tok::l_paren)) {
-    SyntaxParsingContext DiffParamsContext(
-        SyntaxContext, SyntaxKind::DifferentiationParams);
+    SyntaxParsingContext DiffParamsContext(SyntaxContext,
+                                           SyntaxKind::DifferentiabilityParams);
     consumeToken(tok::l_paren);
     // Parse first parameter. At least one is required.
     if (parseParam())
@@ -942,7 +942,7 @@ bool Parser::parseDifferentiabilityParametersClause(
     while (Tok.isNot(tok::r_paren))
       if (parseParam())
         return errorAndSkipUntilConsumeRightParen(*this, attrName, 2);
-    SyntaxContext->collectNodesInPlace(SyntaxKind::DifferentiationParamList);
+    SyntaxContext->collectNodesInPlace(SyntaxKind::DifferentiabilityParamList);
     // Parse closing ')' of the parameter list.
     consumeToken(tok::r_paren);
   }