[clangd] Move buildStaticIndex() to SymbolYAML

`buildStaticIndex()` is used by two other tools that I'm building, now
it's useful outside of `tool/ClangdMain.cpp`.

Also, slightly refactor the code while moving it to the different source
file.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D51626

llvm-svn: 341369