[swiftc (24 vs. 5389)] Add crasher in swift::Parser::parseTopLevel(...)

Add test case for crash triggered in `swift::Parser::parseTopLevel(...)`.

Current number of unresolved compiler crashers: 24 (5389 resolved)

Assertion failure in [`lib/Parse/ParseDecl.cpp (line 222)`](https://github.com/apple/swift/blob/master/lib/Parse/ParseDecl.cpp#L222):

```
Assertion `isInSILMode() && "'sil' should only be a keyword in SIL mode"' failed.

When executing: bool swift::Parser::parseTopLevel()
```

Assertion context:

```
    parseSILDefaultWitnessTable();
  } else if (Tok.is(tok::kw_sil_coverage_map)) {
    assert(isInSILMode() && "'sil' should only be a keyword in SIL mode");
    parseSILCoverageMap();
  } else if (Tok.is(tok::kw_sil_scope)) {
    assert(isInSILMode() && "'sil' should only be a keyword in SIL mode");
    parseSILScope();
  } else {
    parseBraceItems(Items,
                    allowTopLevelCode() ? BraceItemListKind::TopLevelCode
                                        : BraceItemListKind::TopLevelLibrary);
```
Stack trace:

```
0 0x0000000003512118 llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/path/to/swift/bin/swift+0x3512118)
1 0x0000000003512856 SignalHandler(int) (/path/to/swift/bin/swift+0x3512856)
2 0x00007f8ea4f2d3e0 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x113e0)
3 0x00007f8ea365b428 gsignal /build/glibc-Qz8a69/glibc-2.23/signal/../sysdeps/unix/sysv/linux/raise.c:54:0
4 0x00007f8ea365d02a abort /build/glibc-Qz8a69/glibc-2.23/stdlib/abort.c:91:0
5 0x00007f8ea3653bd7 __assert_fail_base /build/glibc-Qz8a69/glibc-2.23/assert/assert.c:92:0
6 0x00007f8ea3653c82 (/lib/x86_64-linux-gnu/libc.so.6+0x2dc82)
7 0x0000000000b80db1 swift::Parser::parseTopLevel() (/path/to/swift/bin/swift+0xb80db1)
8 0x0000000000bb4f10 swift::parseIntoSourceFile(swift::SourceFile&, unsigned int, bool*, swift::SILParserState*, swift::PersistentParserState*, swift::DelayedParsingCallbacks*) (/path/to/swift/bin/swift+0xbb4f10)
9 0x0000000000996713 swift::CompilerInstance::performSema() (/path/to/swift/bin/swift+0x996713)
10 0x000000000047c5aa swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) (/path/to/swift/bin/swift+0x47c5aa)
11 0x000000000043ade7 main (/path/to/swift/bin/swift+0x43ade7)
12 0x00007f8ea3646830 __libc_start_main /build/glibc-Qz8a69/glibc-2.23/csu/../csu/libc-start.c:325:0
13 0x0000000000438229 _start (/path/to/swift/bin/swift+0x438229)
```

Author: practicalswift

validation-test/compiler_crashers/28600-isinsilmode-sil-should-only-be-a-keyword-in-sil-mode.swift

@@ -0,0 +1,10 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// RUN: not --crash %target-swift-frontend %s -emit-ir
+// REQUIRES: asserts
+sil_scope