[clangd] Using symbol name to map includes for STL symbols.

[clangd] Using symbol name to map includes for STL symbols.

Summary:
Using suffix path mapping relies on the STL implementations, and it is
not portable. This patch is using symbol name mapping, which should
work with different STL implementations, fix clangd/clangd#9.

To generate the symbol mapping, we parse the cppreference symbol index
page to build a lookup table.

The mapping is not completed, a few TODOs:
  - support symbols from different headers (e.g. std::move)
  - support STL macros
  - support symbols from std's sub-namespaces (e.g. chrono)

Reviewers: ioeric, jfb, serge-sans-paille

Reviewed By: ioeric

Subscribers: sammccall, klimek, ilya-biryukov, ioeric, MaskRay, jkorous, mgrang, arphaman, kadircet, jfb, jdoerfert, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D58345

llvm-svn: 356134