[clangd] collect symbol #include & insert #include in global code completion.

Summary:
o Collect suitable #include paths for index symbols. This also does smart mapping
for STL symbols and IWYU pragma (code borrowed from include-fixer).
o For global code completion, add a command for inserting new #include in each code
completion item.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, mgorny, ilya-biryukov, jkorous-apple, hintonda, cfe-commits

Differential Revision: https://reviews.llvm.org/D42640

llvm-svn: 325343