clangd: Tolerate additional headers

Summary:
The language server protocol specified 2 headers (Content-Length and Content-Type), but does not specify their sequence. It specifies that an empty line ends
headers. Clangd has been updated to handle arbitrary sequences of headers, extracting only the content length.

Patch by puremourning (Ben Jackson).

Reviewers: bkramer, klimek, ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: cfe-commits, ilya-biryukov

Differential Revision: https://reviews.llvm.org/D37282

llvm-svn: 312483