[clangd] Use string[] for allCommitCharacters

As per LSP specification, allCommitCharacters should be string[] instead of
string:

https://microsoft.github.io/language-server-protocol/specification#textDocument_completion

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D86604

Author: kirillbobyrev

clang-tools-extra/clangd/ClangdLSPServer.cpp

@@ -592,7 +592,10 @@ void ClangdLSPServer::onInitialize(const InitializeParams &Params,
             {"codeActionProvider", std::move(CodeActionProvider)},
             {"completionProvider",
              llvm::json::Object{
-                 {"allCommitCharacters", " \t()[]{}<>:;,+-/*%^&#?.=\"'|"},
+                 {"allCommitCharacters",
+                  {" ", "\t", "(", ")", "[", "]", "{",  "}", "<",
+                   ">", ":",  ";", ",", "+", "-", "/",  "*", "%",
+                   "^", "&",  "#", "?", ".", "=", "\"", "'", "|"}},
                  {"resolveProvider", false},
                  // We do extra checks, e.g. that > is part of ->.
                  {"triggerCharacters", {".", "<", ">", ":", "\"", "/"}},

clang-tools-extra/clangd/test/initialize-params.test

@@ -7,7 +7,35 @@
 # CHECK-NEXT:    "capabilities": {
 # CHECK-NEXT:      "codeActionProvider": true,
 # CHECK-NEXT:      "completionProvider": {
-# CHECK-NEXT:        "allCommitCharacters": " \t()[]{}<>:;,+-/*%^&#?.=\"'|",
+# CHECK-NEXT:        "allCommitCharacters": [
+# CHECK-NEXT:          " ",
+# CHECK-NEXT:          "\t",
+# CHECK-NEXT:          "(",
+# CHECK-NEXT:          ")",
+# CHECK-NEXT:          "[",
+# CHECK-NEXT:          "]",
+# CHECK-NEXT:          "{",
+# CHECK-NEXT:          "}",
+# CHECK-NEXT:          "<",
+# CHECK-NEXT:          ">",
+# CHECK-NEXT:          ":",
+# CHECK-NEXT:          ";",
+# CHECK-NEXT:          ",",
+# CHECK-NEXT:          "+",
+# CHECK-NEXT:          "-",
+# CHECK-NEXT:          "/",
+# CHECK-NEXT:          "*",
+# CHECK-NEXT:          "%",
+# CHECK-NEXT:          "^",
+# CHECK-NEXT:          "&",
+# CHECK-NEXT:          "#",
+# CHECK-NEXT:          "?",
+# CHECK-NEXT:          ".",
+# CHECK-NEXT:          "=",
+# CHECK-NEXT:          "\"",
+# CHECK-NEXT:          "'",
+# CHECK-NEXT:          "|"
+# CHECK-NEXT:        ],
 # CHECK-NEXT:        "resolveProvider": false,
 # CHECK-NEXT:        "triggerCharacters": [
 # CHECK-NEXT:          ".",