[ThinLTO] Escape module paths when printing

We have located a bug in AssemblyWriter::printModuleSummaryIndex(). This
function outputs path strings incorrectly. Backslashes in the strings
are not correctly escaped.

Consequently, if a path name contains a backslash followed by two
hexadecimal characters, the sequence is incorrectly interpreted when the
output is read by another component. This mangles the path and results
in error.

This patch fixes this issue by calling printEscapedString() to output
the module paths.

Patch by Chris Jackson.

Differential Revision: https://reviews.llvm.org/D49090

llvm-svn: 336908