[clangd] Detect C++ for extension-less source files in vscode extension

[hokein]

Summary

Extend our extension to support detecting these files as C++ files based on the first
line (-*- C++ -*-), it will make clangd work on C++ standard headers
(e.g. iostream).

We use the contributes.languages[1] to enrich the buildin VScode C++
support.

[1]: https://code.visualstudio.com/api/references/contribution-points#contributes.languages

[kadircet]

Could you also update summary to include a link to documentation regarding contributes.languages ?

clang-tools-extra/clangd/clients/clangd-vscode/package.json

55 ↗

(On Diff #204997)

could you also make sure it starts with "^//"

[hokein]

Address comments.

[malaperle]

"Extend our extension to support detecting these files as C++ files based on the first
line (-*- C++ -*-), it will make clangd work on C++ standard headers
(e.g. iostream)."

This is not in the standard right? I don't see this in MSVC, so it wouldn't work there.

[malaperle]

In D63397#1696000, @malaperle wrote:

"Extend our extension to support detecting these files as C++ files based on the first
line (-*- C++ -*-), it will make clangd work on C++ standard headers
(e.g. iostream)."

This is not in the standard right? I don't see this in MSVC, so it wouldn't work there.

Never mind, I just saw r363662

clang-tools-extra/trunk/clangd/clients/clangd-vscode/package.json

"devDependencies": {
         "typescript": "^2.0.3",
         "vscode": "^1.1.0"
     },
     "repository": {
         "type": "svn",
         "url": "http://llvm.org/svn/llvm-project/clang-tools-extra/trunk/clangd/clients/clangd-vscode/"
     },
     "contributes": {
+        "languages": [{
+            "id": "cpp",
+            "firstLine": "^\/[/*].*-\\*-\\s*C\\+\\+\\s*-\\*-.*"
+        }],
         "configuration": {
             "type": "object",
             "title": "clangd configuration",
             "properties": {
                 "clangd.path": {
                     "type": "string",
                     "default": "clangd",
                     "description": "The path to clangd executable, e.g.: /usr/bin/clangd"