[lldb] build.py: fix behavior when passing --compiler=/path/to/compiler

All the other paths in the find_toolchain function return a tuple
(detected_toolchain_type, compiler_path), but when the parameter to
--compiler is not one of the predefined names it only returns the
detected toolchain type, which causes an error when trying to unpack the
result.

This patch changes it to return also the compiler path passed as a
parameter.

Differential Revision: https://reviews.llvm.org/D61713

llvm-svn: 360355