[Clangd] ExtractFunction: Don't extract body of enclosing function.

Summary:
This patch disable extraction of the body of the enclosing function.
`void f() [[{}]]`

Extracting this CompoundStmt would leave the enclosing function without
a body.

Reviewers: sammccall, kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68245

llvm-svn: 373472