[Parse] Improve diagnostic and add fixit to correct '#else if' to '#elseif'

include/swift/AST/DiagnosticsParse.def

@@ -68,6 +68,10 @@ ERROR(extra_tokens_conditional_compilation_directive,none,
       "extra tokens following conditional compilation directive", ())
 ERROR(unexpected_rbrace_in_conditional_compilation_block,none,
       "unexpected '}' in conditional compilation block", ())
+ERROR(unexpected_if_following_else_compilation_directive,none,
+      "unexpected 'if' keyword following '#else' conditional compilation "
+      "directive; did you mean '#elseif'?",
+      ())
 
 ERROR(pound_diagnostic_expected_string,none,
       "expected string literal in %select{#warning|#error}0 directive",(bool))

lib/Parse/ParseIfConfig.cpp

@@ -594,6 +594,12 @@ ParserResult<IfConfigDecl> Parser::parseIfConfig(
     Expr *Condition = nullptr;
     bool isActive = false;
 
+    if (!Tok.isAtStartOfLine() && isElse && Tok.is(tok::kw_if)) {
+      diagnose(Tok, diag::unexpected_if_following_else_compilation_directive)
+          .fixItReplace(SourceRange(ClauseLoc, consumeToken()), "#elseif");
+      isElse = false;
+    }
+
     // Parse the condition.  Evaluate it to determine the active
     // clause unless we're doing a parse-only pass.
     if (isElse) {

test/Parse/ConditionalCompilation/basicParseErrors.swift

@@ -138,3 +138,14 @@ fn_k()
 func undefinedFunc() // ignored.
 #endif
 undefinedFunc() // expected-error {{use of unresolved identifier 'undefinedFunc'}}
+
+#if FOO
+#else if BAR
+// expected-error@-1 {{unexpected 'if' keyword following '#else' conditional compilation directive; did you mean '#elseif'?}} {{1-9=#elseif}}
+#else
+#endif
+
+#if FOO
+#else
+if true {}
+#endif // OK