[SourceKit] Add test case for crash triggered in swift::Parser::parseDeclVar(swift::OptionSet<swift::Parser::ParseDeclFlags, unsigned int>, swift::DeclAttributes&, llvm::SmallVectorImpl<swift::Decl*>&, swift::SourceLoc, swift::StaticSpellingKind, swift::SourceLoc)

Stack trace:

```
found code completion token A at offset 123
swift-ide-test: /path/to/swift/include/swift/Parse/ParserResult.h:73: T *swift::ParserResult<swift::Expr>::get() const [T = swift::Expr]: Assertion `getPtrOrNull() && "not checked for nullptr"' failed.
8  swift-ide-test  0x000000000087687b swift::Parser::parseDeclVar(swift::OptionSet<swift::Parser::ParseDeclFlags, unsigned int>, swift::DeclAttributes&, llvm::SmallVectorImpl<swift::Decl*>&, swift::SourceLoc, swift::StaticSpellingKind, swift::SourceLoc) + 4619
9  swift-ide-test  0x00000000008737c4 swift::Parser::parseDecl(llvm::SmallVectorImpl<swift::Decl*>&, swift::OptionSet<swift::Parser::ParseDeclFlags, unsigned int>) + 2836
11 swift-ide-test  0x00000000008680c8 swift::Parser::parseList(swift::tok, swift::SourceLoc, swift::SourceLoc&, swift::tok, bool, bool, swift::Diag<>, std::function<swift::ParserStatus ()>) + 728
12 swift-ide-test  0x000000000087a612 swift::Parser::parseDeclProtocol(swift::OptionSet<swift::Parser::ParseDeclFlags, unsigned int>, swift::DeclAttributes&) + 1602
13 swift-ide-test  0x0000000000873a77 swift::Parser::parseDecl(llvm::SmallVectorImpl<swift::Decl*>&, swift::OptionSet<swift::Parser::ParseDeclFlags, unsigned int>) + 3527
14 swift-ide-test  0x00000000008bebab swift::Parser::parseBraceItems(llvm::SmallVectorImpl<swift::ASTNode>&, swift::BraceItemListKind, swift::BraceItemListKind) + 2027
15 swift-ide-test  0x000000000086a30c swift::Parser::parseTopLevel() + 156
16 swift-ide-test  0x000000000086591b swift::parseIntoSourceFile(swift::SourceFile&, unsigned int, bool*, swift::SILParserState*, swift::PersistentParserState*, swift::DelayedParsingCallbacks*) + 107
17 swift-ide-test  0x0000000000774176 swift::CompilerInstance::performSema() + 2918
18 swift-ide-test  0x000000000071cc33 main + 35011
Stack dump:
0.	Program arguments: swift-ide-test -code-completion -code-completion-token=A -source-filename=<INPUT-FILE>
1.	With parser at source location: <INPUT-FILE>:2:20
```

Author: practicalswift

validation-test/IDE/crashers/014-swift-parser-parsedeclvar.swift

@@ -0,0 +1,2 @@
+// RUN: not --crash %target-swift-ide-test -code-completion -code-completion-token=A -source-filename=%s
+protocol A{let h=o#^A^#