[XRay][llvm] Load XRay Profiles

Summary:
This change implements the profile loading functionality in LLVM to
support XRay's profiling mode in compiler-rt.

We introduce a type named `llvm::xray::Profile` which allows building a
profile representation. We can load an XRay profile from a file to build
Profile instances, or do it manually through the Profile type's API.

The intent is to get the `llvm-xray` tool to generate `Profile`
instances and use that as the common abstraction through which all
conversion and analysis can be done. In the future we can generate
`Profile` instances from `Trace` instances as well, through conversion
functions.

Some of the key operations supported by the `Profile` API are:

- Path interning (`Profile::internPath(...)`) which returns a unique path
  identifier.

- Block appending (`Profile::addBlock(...)`) to add thread-associated
  profile information.

- Path ID to Path lookup (`Profile::expandPath(...)`) to look up a
  PathID and return the original interned path.

- Block iteration.

A 'Path' in this context represents the function call stack in
leaf-to-root order. This is represented as a path in an internally
managed prefix tree in the `Profile` instance. Having a handle (PathID)
to identify the unique Paths we encounter for a particular Profile
allows us to reduce the amount of memory required to associate profile
data to a particular Path.

This is the first of a series of patches to migrate the `llvm-stacks`
tool towards using a single profile representation.

Depends on D48653.

Reviewers: kpw, eizan

Reviewed By: kpw

Subscribers: kpw, thakis, mgorny, llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D48370

llvm-svn: 341012

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