Handle extension contexts when demangling bound generic arguments

[bitjammer]

Local generic types can appear inside functions inside extensions
of other types. When demangling bound generic arguments, the demangler
assumed that a module was the only other kind of context outside
of nominal types.

rdar://problem/27573079

[bitjammer]

@swift-ci Please test

[tkremenek]

CI fails:

/Users/buildnode/jenkins/workspace/swift-PR-osx/swift/stdlib/public/SDK/Foundation/Notification.swift:134:16: warning: explicitly specified type '[String : Any]?' adds an additional level of optional to the initializer, making the optional check always succeed
            if let info : [String : Any]? = _NSUserInfoDictionary.bridgeReference(from: userInfo) {
               ^          ~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                          [String : Any]
Assertion failed: (srcAddr->getType() == destAddr->getType()), function createCopyAddr, file /Users/buildnode/jenkins/workspace/swift-PR-osx/swift/include/swift/SIL/SILBuilder.h, line 516.
0  swift                    0x000000010b02a71b llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 43
1  swift                    0x000000010b029946 llvm::sys::RunSignalHandlers() + 70
2  swift                    0x000000010b02aff1 SignalHandler(int) + 641
3  libsystem_platform.dylib 0x00007fff966d252a _sigtramp + 26
4  swift                    0x000000010bc255e1 cmark_strbuf__initbuf + 71488
5  swift                    0x000000010b02ac3e abort + 14
6  swift                    0x000000010b02ac21 __assert_rtn + 81
7  swift                    0x000000010872adcd swift::SILBuilder::createCopyAddr(swift::SILLocation, swift::SILValue, swift::SILValue, swift::IsTake_t, swift::IsInitialization_t) + 333
8  swift                    0x00000001084bddd7 swift::SILCloner<swift::SILInliner>::visitCopyAddrInst(swift::CopyAddrInst*) + 343
9  swift                    0x00000001084ba11b swift::SILCloner<swift::SILInliner>::visitSILBasicBlock(swift::SILBasicBlock*) + 91
10 swift                    0x00000001084b9bb6 swift::SILInliner::inlineFunction(swift::FullApplySite, llvm::ArrayRef<swift::SILValue>) + 1206
11 swift                    0x000000010857d730 (anonymous namespace)::SILPerformanceInliner::inlineCallsIntoFunction(swift::SILFunction*) + 12112
12 swift                    0x000000010857a6ba (anonymous namespace)::SILPerformanceInlinerPass::run() + 410
13 swift                    0x00000001084d22cf swift::SILPassManager::runPassesOnFunction(llvm::ArrayRef<swift::SILFunctionTransform*>, swift::SILFunction*, bool) + 2255
14 swift                    0x00000001084d32f9 swift::SILPassManager::runFunctionPasses(llvm::ArrayRef<swift::SILFunctionTransform*>) + 1193
15 swift                    0x00000001084d4634 swift::SILPassManager::runOneIteration() + 964
16 swift                    0x00000001084df317 swift::runSILOptimizationPasses(swift::SILModule&) + 231
17 swift                    0x00000001081d1acf performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*) + 13903
18 swift                    0x00000001081cd1af swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2895
19 swift                    0x000000010818fb00 main + 2448
20 libdyld.dylib            0x00007fff879795ad start + 1

[tkremenek]

I'd prefer we just pick this fix up when we pull a seed branch again from master.