Handle inline stacks in gcov-encoded sample profiles.

This patch adds support for reading sample profiles with inline stacks.
Inline stacks in a profile are generated when the sampled binary has
samples in inlined functions.

For instance, if main() calls foo() and foo() calls bar(), and bar() is
inlined into foo() and foo() inlined into main(), the profile may look
something like:

main total:364084 head:0
  [ ... ]
  2.3: _Z3fool total:243786
    1: 60149
    1.2: 38568
    1.4: 46511
    1.7: _Z3bari total:98558
      1.1: 52672
      1.2: 45886

At line 2, discriminator 3, main() calls foo(). In turn, foo() calls
bar() at line 1, discriminator 7.

In the textual format, this stacking of inline calls is represented
with indentation.

With this change, LLVM can now read sample profile files generated by
the create_gcov tool from https://github.com/google/autofdo.

llvm-svn: 249644