[Parse] Let #if diretives be parsed in parseBraceItems()

parseBraceItems() has specific logic for pasing conditional compilation blocks.
Withoutout that, decralations in the block cannot be propagated to outside.
For instance:

  FOO: #if true
  func foo() {}
  #endif
  foo() // error: use of unresolved identifier 'foo'

Author: rintaro

lib/Parse/ParseStmt.cpp

@@ -522,6 +522,7 @@ ParserResult<Stmt> Parser::parseStmt() {
   switch (Tok.getKind()) {
   case tok::pound_line:
   case tok::pound_sourceLocation:
+  case tok::pound_if:
     assert((LabelInfo || tryLoc.isValid()) &&
            "unlabeled directives should be handled earlier");
     // Bailout, and let parseBraceItems() parse them.
@@ -546,10 +547,6 @@ ParserResult<Stmt> Parser::parseStmt() {
     if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
     if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
     return parseStmtGuard();
-  case tok::pound_if:
-    if (LabelInfo) diagnose(LabelInfo.Loc, diag::invalid_label_on_stmt);
-    if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
-    return parseStmtIfConfig();
   case tok::kw_while:
     if (tryLoc.isValid()) diagnose(tryLoc, diag::try_on_stmt, Tok.getText());
     return parseStmtWhile(LabelInfo);

test/Parse/ConditionalCompilation/basicParseErrors.swift

@@ -64,3 +64,14 @@ struct S {
 
 #if _endian(mid) // expected-warning {{unknown endianness for build configuration '_endian'}}
 #endif
+
+LABEL: #if true // expected-error {{expected statement}}
+func fn_i() {}
+#endif
+fn_i() // OK
+
+try #if false // expected-error {{expected expression}}
+#else
+func fn_j() {}
+#endif
+fn_j() // OK