[llvm-profdata] Change instr prof counter overflow to saturate rather than discard

Summary: This changes overflow handling during instrumentation profile merge. Rathar than throwing away records that would result in counter overflow, merged counts are instead clamped to the maximum representable value. A warning about counter overflow is still surfaced to the user as before.

Reviewers: dnovillo, davidxl, silvas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14893

llvm-svn: 254525