Move valid caller-pc checks out of platform-specific checks

Summary:
ProcessPlatformSpecificAllocations for linux leak sanitizer iterated over
memory chunks and ran two checks concurrently:
1) Ensured the pc was valid
2) Checked whether it was a linker allocation

All platforms will need the valid pc check, so it is moved out of the platform-
specific file. To prevent code and logic duplication, the linker allocation
check is moved as well, with the name of the linker supplied by the platform-specific
module. In cases where we don't need to check for linker allocations (ie Darwin),
this name will be a nullptr, and we'll only run the caller pc checks.

Reviewers: kubamracek, alekseyshl, kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32130

llvm-svn: 300690