Repository: TypeScript
Issue: [Bug] Occurrence Highlight does not work in case of file path case mismatch
Labels: Bug,Help Wanted

### Issue:
On a TypeScript project, if the TSConfig has relative files paths, Occurrence Highlight in Editor works for that file
If it has absolute paths with upper-case Drive letter, it doesn't.

### Debugging
I checked TS Server logs for 2 cases:
1. File name in tsConfig is "./test.ts" (Occurrence Highlight **works**)
2. File name in tsConfig is "Z:/Code/Playground/test-ts-absolutePaths/test.ts" (Occurrence Highlight **doesn't work**)

In case 1, the documentHighlights command Request and Response are
```
{
	"seq": 13,
	"type": "request",
	"command": "documentHighlights",
	"arguments": {
		"file": "z:/Code/Playground/test-ts-absolutePaths/test.ts",
		"line": 2,
		"offset": 15,
		"filesToSearch": [
			"z:/Code/Playground/test-ts-absolutePaths/test.ts"
		]
	}
}

{
	"seq": 0,
	"type": "response",
	"command": "documentHighlights",
	"request_seq": 13,
	"success": true,
	"body": [
		{
			"file": "z:/Code/Playground/test-ts-absolutePaths/test.ts",
			"highlightSpans": [
				{
					"start": {
						"line": 2,
						"offset": 13
					},
					"end": {
						"line": 2,
						"offset": 17
					},
					"contextStart": {
						"line": 2,
						"offset": 5
					},
					"contextEnd": {
						"line": 2,
						"offset": 26
					},
					"kind": "reference"
				},
				{
					"start": {
						"line": 5,
						"offset": 14
					},
					"end": {
						"line": 5,
						"offset": 18
					},
					"kind": "writtenReference"
				},
				{
					"start": {
						"line": 9,
						"offset": 26
					},
					"end": {
						"line": 9,
						"offset": 30
					},
					"kind": "reference"
				}
			]
		}
	]
}
```

In case 2
```
{
	"seq": 18,
	"type": "request",
	"command": "documentHighlights",
	"arguments": {
		"file": "z:/Code/Playground/test-ts-absolutePaths/test.ts",
		"line": 2,
		"offset": 15,
		"filesToSearch": [
			"z:/Code/Playground/test-ts-absolutePaths/test.ts"
		]
	}
}

{
	"seq": 0,
	"type": "response",
	"command": "documentHighlights",
	"request_seq": 18,
	"success": true,
	"body": [
		{
			"file": "Z:/Code/Playground/test-ts-absolutePaths/test.ts",
			"highlightSpans": [
				{
					"start": {
						"line": 2,
						"offset": 13
					},
					"end": {
						"line": 2,
						"offset": 17
					},
					"contextStart": {
						"line": 2,
						"offset": 5
					},
					"contextEnd": {
						"line": 2,
						"offset": 26
					},
					"kind": "reference"
				},
				{
					"start": {
						"line": 5,
						"offset": 14
					},
					"end": {
						"line": 5,
						"offset": 18
					},
					"kind": "writtenReference"
				},
				{
					"start": {
						"line": 9,
						"offset": 26
					},
					"end": {
						"line": 9,
						"offset": 30
					},
					"kind": "reference"
				}
			]
		}
	]
}
```

The only difference is the Drive letter case (Z vs z)
TS Server seems to work as intended.
I can see 2 possible solutions:
- When receiving response, Occurrence Highlight should use case-insensitive file path matching between request.arguments.filesToSearch[0] and response.body[0].file
- When sending request, Occurrence Highlight should send the file path with an upper-case Drive letter (This is not solving other cases where TsConfig has the case mismatch in filename)

Project folder is zipped and attached
[test-ts-absolutePaths.zip](https://github.com/microsoft/vscode/files/6108162/test-ts-absolutePaths.zip)