[BPF] Use SectionForGlobal() for section names computation in BTF

Use function TargetLoweringObjectFile::SectionForGlobal() to compute
section names for globals described in BTF_KIND_DATASEC records.

This fixes a discrepancy in section name computation between
BTFDebug::processGlobals and the rest of the LLVM pipeline.

Specifically, the following example illustrates the discrepancy
before this commit:

  struct Foo {
    int i;
  } __attribute__((aligned(16)));
  struct Foo foo = { 0 };

The initializer for 'foo' looks as follows:

  %struct.Foo { i32 0, [12 x i8] undef }

TargetLoweringObjectFile::SectionForGlobal() classifies 'foo' as
a part of '.bss' section, while BTFDebug::processGlobals
classified it as a part of '.data' section because of the
following expression:

  SecName = Global.getInitializer()->isZeroValue() ? ".bss" : ".data"

The isZeroValue() returns false because of the undef tail of the
initializer, while SectionForGlobal() allows such patterns in '.bss'.

Differential Revision: https://reviews.llvm.org/D140505