[DebugInfo] Fix fragment offset emission order for symbol locations

Summary:
When emitting the location for a global variable with fragmented debug
expressions, make sure that the offset pieces, which represent
optimized-out parts of the variable, are emitted before their succeeding
fragments' expressions. Previously, if the succeeding fragment's
location was a symbol, the offset piece was emitted after, rather than
before, that symbol's expression. This effectively meant that the symbols
were associated with the wrong parts of the variable.

This fixes PR36085.

Patch by: David Stenberg

Reviewers: aprantl, probinson, dblaikie

Reviewed By: aprantl

Subscribers: JDevlieghere, llvm-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D42527

llvm-svn: 323644