getMainExecutable: handle realpath() failure, falling back to getprogpath().

Summary:
Previously, we'd pass a nullptr to std::string and crash().

This case happens when the binary is deleted while being used (e.g. rebuilding clangd).

Reviewers: kadircet

Subscribers: ilya-biryukov, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64068

llvm-svn: 364936