Completions crash in call to `new Map(...)`.

[DanielRosenwasser]

const m = new Map([
    [/*a*/'0', ['0', false]]/*b*/,
]);

Request completions at /*a*/ or /*b*/.

[error] panic handling requesttextDocument/completionruntime error: index out of range [-1]goroutine 3826 [running]:
runtime/debug.Stack()
	runtime/debug/stack.go:26 +0x64
github.com/microsoft/typescript-go/internal/lsp.(*Server).recover(0x4000198008, 0x4019948ff0)
	github.com/microsoft/typescript-go/internal/lsp/server.go:872 +0x40
panic({0x7ff7c70f3b80?, 0x40177be4b0?})
	runtime/panic.go:783 +0x120
github.com/microsoft/typescript-go/internal/checker.(*Checker).getContextualTypeForElementExpression.func1(0x401a5d2ee0)
	github.com/microsoft/typescript-go/internal/checker/checker.go:28915 +0x29c
github.com/microsoft/typescript-go/internal/checker.(*Checker).mapTypeEx(0x4005641e60?, 0x4018009db0?, 0x40178d3158?, 0x94?)
	github.com/microsoft/typescript-go/internal/checker/checker.go:24914 +0xd4
github.com/microsoft/typescript-go/internal/checker.(*Checker).getContextualTypeForElementExpression(0x40106ea608?, 0x4018009d60?, 0x2?, 0x4?, 0x401978a6c0?, 0x7ff7c73b4af8?)
	github.com/microsoft/typescript-go/internal/checker/checker.go:28910 +0x3c
github.com/microsoft/typescript-go/internal/checker.(*Checker).getContextualType(0x40106ea608, 0x401978a6c0, 0x4)
	github.com/microsoft/typescript-go/internal/checker/checker.go:28328 +0x1e0
github.com/microsoft/typescript-go/internal/checker.(*Checker).GetContextualType.func1()
	github.com/microsoft/typescript-go/internal/checker/services.go:310 +0x24
github.com/microsoft/typescript-go/internal/checker.runWithoutResolvedSignatureCaching[...](0x40106ea608?, 0x4018009db0, 0x400083f4a8?)
	github.com/microsoft/typescript-go/internal/checker/services.go:365 +0x2f0
github.com/microsoft/typescript-go/internal/checker.runWithInferenceBlockedFromSourceNode[...](0x40106ea608?, 0x401978a6c0, 0x400083f4a8?)
	github.com/microsoft/typescript-go/internal/checker/services.go:329 +0xe4
github.com/microsoft/typescript-go/internal/checker.(*Checker).GetContextualType(0x400083f620?, 0x400083f5c0?, 0x83f574?)
	github.com/microsoft/typescript-go/internal/checker/services.go:310 +0x48
github.com/microsoft/typescript-go/internal/ls.getContextualType(0x401978a6c0, 0x400c205808?, 0x40106ea608?, 0x40106ea608)
	github.com/microsoft/typescript-go/internal/ls/completions.go:2995 +0x2a0
github.com/microsoft/typescript-go/internal/ls.(*LanguageService).getCompletionData(0x40097bbe90, {0x7ff7c74e2b88, 0x40097bbda0}, 0x40106ea608, 0x400c205808, 0xb8, 0x400326e800)
	github.com/microsoft/typescript-go/internal/ls/completions.go:1727 +0x1038
github.com/microsoft/typescript-go/internal/ls.(*LanguageService).getCompletionsAtPosition(0x40097bbe90, {0x7ff7c74e2b88, 0x40097bbda0}, 0x400c205808, 0xb8, 0x0)
	github.com/microsoft/typescript-go/internal/ls/completions.go:383 +0x22c
github.com/microsoft/typescript-go/internal/ls.(*LanguageService).ProvideCompletion(0x40097bbe90, {0x7ff7c74e2b88, 0x40097bbda0}, {0x400dc60000?, 0x40097bbda0?}, {0xdc60000?, 0x40?}, 0x4012c7a830)
	github.com/microsoft/typescript-go/internal/ls/completions.go:44 +0xa0
github.com/microsoft/typescript-go/internal/lsp.(*Server).handleCompletion(0x400443fe78?, {0x7ff7c74e2b88?, 0x40097bbda0?}, 0x400dc60000?, 0x400443fee8?)
	github.com/microsoft/typescript-go/internal/lsp/server.go:1208 +0x3c
github.com/microsoft/typescript-go/internal/lsp.init.func1.registerLanguageServiceDocumentRequestHandler[...].16({0x7ff7c74e2b88, 0x40097bbda0}, 0x4019948ff0)
	github.com/microsoft/typescript-go/internal/lsp/server.go:621 +0xec
github.com/microsoft/typescript-go/internal/lsp.(*Server).handleRequestOrNotification(0x4000198008, {0x7ff7c74e2bc0?, 0x401e249220?}, 0x4019948ff0)
	github.com/microsoft/typescript-go/internal/lsp/server.go:502 +0x128
github.com/microsoft/typescript-go/internal/lsp.(*Server).dispatchLoop.func1()
	github.com/microsoft/typescript-go/internal/lsp/server.go:405 +0x34
created by github.com/microsoft/typescript-go/internal/lsp.(*Server).dispatchLoop in goroutine 20
	github.com/microsoft/typescript-go/internal/lsp/server.go:425 +0x7a0