[clang][clangd] Ensure the stack bottom before building AST

`clang::runWithSufficientStackSpace` requires the address of the
initial stack bottom to prevent potential stack overflows.

In addition, add a fallback to ASTFrontendAction in case any client
forgets to call it when not through CompilerInstance::ExecuteAction,
which is rare.

Fixes https://github.com/clangd/clangd/issues/1745.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D158967