Tests for overflow when merging instrumented profiles.

1- Merge profile having maximum counts with itself and verify overflow detected and saturation occurred
RUN: llvm-profdata merge -instr %p/Inputs/overflow-instr.proftext %p/Inputs/overflow-instr.proftext -o %t.out 2>&1 | FileCheck %s -check-prefix=MERGE_OVERFLOW
RUN: llvm-profdata show -instr -all-functions -counts %t.out | FileCheck %s --check-prefix=SHOW_OVERFLOW
MERGE_OVERFLOW: {{.*}}.proftext: overflow: counter overflow
SHOW_OVERFLOW: Function count: 18446744073709551615
SHOW_OVERFLOW-NEXT: Block counts: [18446744073709551615, 18446744073709551615]

2- Merge profile having maximum counts by itself and verify no overflow
RUN: llvm-profdata merge -instr %p/Inputs/overflow-instr.proftext -o %t.out 2>&1 | FileCheck %s -check-prefix=MERGE_NO_OVERFLOW -allow-empty
RUN: llvm-profdata show -instr -all-functions -counts %t.out | FileCheck %s --check-prefix=SHOW_NO_OVERFLOW
MERGE_NO_OVERFLOW-NOT: {{.*}}.proftext: overflow: counter overflow
SHOW_NO_OVERFLOW: Function count: 18446744073709551615
SHOW_NO_OVERFLOW-NEXT: Block counts: [9223372036854775808, 18446744073709551615]