[clangd] Fix crash when CompilerInvocation can't be created.

Summary:
This can happen if the CompileCommand provided by compilation database
is malformed.

Reviewers: hokein, ioeric, sammccall

Reviewed By: sammccall

Subscribers: klimek, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43122

llvm-svn: 324732