[SourceKit] Add test case for crash triggered in llvm::llvm_unreachable_internal(char const*, char const*, unsigned int)

Stack trace:

```
found code completion token A at offset 158
Polymorphic function type should have been opened
UNREACHABLE executed at /path/to/swift/lib/Sema/CSSimplify.cpp:1524!
6  swift-ide-test  0x0000000002a6564d llvm::llvm_unreachable_internal(char const*, char const*, unsigned int) + 461
7  swift-ide-test  0x00000000008d8cde swift::constraints::ConstraintSystem::matchTypes(swift::Type, swift::Type, swift::constraints::TypeMatchKind, unsigned int, swift::constraints::ConstraintLocatorBuilder) + 15374
8  swift-ide-test  0x00000000008e1a9f swift::constraints::ConstraintSystem::simplifyConstraint(swift::constraints::Constraint const&) + 639
9  swift-ide-test  0x00000000009a5f67 swift::constraints::ConstraintSystem::addConstraint(swift::constraints::Constraint*, bool, bool) + 23
10 swift-ide-test  0x0000000000919983 swift::TypeChecker::typesSatisfyConstraint(swift::Type, swift::Type, swift::constraints::ConstraintKind, swift::DeclContext*) + 131
11 swift-ide-test  0x00000000008ff59b swift::isConvertibleTo(swift::Type, swift::Type, swift::DeclContext*) + 75
13 swift-ide-test  0x000000000077406c swift::ide::CodeCompletionResultBuilder::takeResult() + 1324
17 swift-ide-test  0x0000000000b576c6 swift::lookupVisibleDecls(swift::VisibleDeclConsumer&, swift::DeclContext const*, swift::LazyResolver*, bool, swift::SourceLoc) + 278
28 swift-ide-test  0x0000000000ae2154 swift::Decl::walk(swift::ASTWalker&) + 20
29 swift-ide-test  0x0000000000b6be0e swift::SourceFile::walk(swift::ASTWalker&) + 174
30 swift-ide-test  0x0000000000b6b03f swift::ModuleDecl::walk(swift::ASTWalker&) + 79
31 swift-ide-test  0x0000000000b451a2 swift::DeclContext::walkContext(swift::ASTWalker&) + 146
32 swift-ide-test  0x000000000085cd1a swift::performDelayedParsing(swift::DeclContext*, swift::PersistentParserState&, swift::CodeCompletionCallbacksFactory*) + 138
33 swift-ide-test  0x000000000076bb24 swift::CompilerInstance::performSema() + 3316
34 swift-ide-test  0x00000000007152b7 main + 33239
Stack dump:
0.	Program arguments: swift-ide-test -code-completion -code-completion-token=A -source-filename=<INPUT-FILE>
1.	While walking into decl declaration 0x5d810b0 at <INPUT-FILE>:3:1
```

Author: practicalswift

validation-test/IDE/crashers/050-swift-constraints-constraintsystem-matchtypes.swift

@@ -0,0 +1,4 @@
+// RUN: not --crash %target-swift-ide-test -code-completion -code-completion-token=A -source-filename=%s
+// REQUIRES: asserts
+{func a<H
+enum b{func a<e{return#^A^#