[clangd] Cleanup: use index file instead of header in workspace symbols lit test.

[ioeric]

Summary

The full path of the input header depends on the execution environment
and may result in different behavior (e.g. when different URI schemes are used).

test/clangd/Inputs/sstream.h

-namespace std {
-class basic_ostringstream {};
-}

test/clangd/Inputs/symbols.test.yaml

+---
+!Symbol
+ID:              057557CEBF6E6B2D
+Name:            'vector'
+Scope:           'std::'
+SymInfo:         
+  Kind:            Class
+  Lang:            Cpp
+CanonicalDeclaration: 
+  FileURI:         'file:///vector.h'
+  Start:           
+    Line:            215
+    Column:          10
+  End:             
+    Line:            215
+    Column:          16
+...

test/clangd/symbols.test

-# RUN: env CPATH=%S/Inputs clangd -lit-test < %s | FileCheck %s
+# RUN: clangd --index-file=%S/Inputs/symbols.test.yaml -lit-test < %s | FileCheck %s
 {"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{"workspace":{"symbol":{"symbolKind":{"valueSet": [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]}}}},"trace":"off"}}
 ---
-{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"test:///main.cpp","languageId":"cpp","version":1,"text":"#include <sstream.h>\nvoid foo(); int main() { foo(); }\n"}}}
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"test:///main.cpp","languageId":"cpp","version":1,"text":"void foo(); int main() { foo(); }\n"}}}
 ---
-{"jsonrpc":"2.0","id":1,"method":"workspace/symbol","params":{"query":"std::basic_ostringstream"}}
+{"jsonrpc":"2.0","id":1,"method":"workspace/symbol","params":{"query":"vector"}}
 #      CHECK:  "id": 1,
 # CHECK-NEXT:  "jsonrpc": "2.0",
 # CHECK-NEXT:    "result": [
 # CHECK-NEXT:      {
 # CHECK-NEXT:        "containerName": "std",
 # CHECK-NEXT:        "kind": 5,
 # CHECK-NEXT:        "location": {
 # CHECK-NEXT:          "range": {
 # CHECK-NEXT:            "end": {
 # CHECK-NEXT:              "character": {{.*}},
 # CHECK-NEXT:              "line": {{.*}}
 # CHECK-NEXT:            },
 # CHECK-NEXT:            "start": {
 # CHECK-NEXT:              "character": {{.*}},
 # CHECK-NEXT:              "line": {{.*}}
 # CHECK-NEXT:            }
 # CHECK-NEXT:          },
-# CHECK-NEXT:          "uri": "file://{{.*}}/sstream.h"
+# CHECK-NEXT:          "uri": "file:///vector.h"
 # CHECK-NEXT:        },
-# CHECK-NEXT:        "name": "basic_ostringstream"
+# CHECK-NEXT:        "name": "vector"
 # CHECK-NEXT:      }
 # CHECK-NEXT:    ]
 # CHECK-NEXT:}
 ---
 {"jsonrpc":"2.0","id":2,"method":"textDocument/documentSymbol","params":{"textDocument":{"uri":"test:///main.cpp"}}}
 #      CHECK:  "id": 2,
 # CHECK-NEXT:  "jsonrpc": "2.0",
 # CHECK-NEXT:    "result": [