Set hidden attribute on lprofMergeValueProfData

Summary:
The changes in https://reviews.llvm.org/D44847 cause load time failure
due to lprofMergeValueProfData in Android libs enabled with profile generation:

"dlopen failed: cannot locate symbol "lprofMergeValueProfData" referenced by..."

Marking lprofMergeValueProfData as hidden so the correct in-module definition
 is picked by the linker.

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: efriedma, xur, davidxl, llvm-commits

Differential Revision: https://reviews.llvm.org/D55893

llvm-svn: 354064