[lit] Implement non-pipelined echo commands internally
[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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@307668 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm