[lit] Implement non-pipelined echo commands internally

Summary:
This speeds up the LLD test suite on Windows by 3x. Most of the time is
spent on lld/test/ELF/linkerscript/diagnostics.s, which repeatedly
constructs linker scripts with appending echo commands.

Reviewers: dlj, zturner, modocache

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35093

llvm-svn: 307668