lldb-test: Improve newline handling

Summary:
Previously lldb-test's LinePrinter would output the indentation spaces
even on completely empty lines. This is not nice, as trailing spaces get
flagged as errors in some tools/editors, and it prevents FileCheck's
CHECK-EMPTY from working.

Equally annoying was the fact that the LinePrinter did not terminate
it's output with a newline (instead it would leave the unterminated hanging
indent from the last NewLine() command), which meant that the shell prompt
following the lldb-test command came out wrong.

This fixes both issues by changing how newlines are handled. NewLine(),
which was ending the previous line ('\n') *and* begging the next line by
printing the indent, is now "demoted" to just printing literal "\n".
Instead, lines are now delimited via a helper Line object, which makes
sure the line is indented and terminated in an RAII fashion. The typical
usage would be:
Printer.line() << "This text will be indented and terminated";
If one needs to do more work than it will fit into a single statement,
one can also assign the result of the line() function to a local
variable. The line will then be terminated when that object goes out of
scope.

Reviewers: zturner

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D55597

llvm-svn: 349269