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[NativePDB] Add the ability to dump dump global variables.

Description

[NativePDB] Add the ability to dump dump global variables.

LLDB has the ability to display global variables, even without a running
process, via the target variable command. This is because global
variables are linker initialized, so their values are embedded directly
into the executables. This gives us great power for testing native PDB
functionality in a cross-platform manner, because we don't actually need
a running process. We can just create a target using an EXE file, and
display global variables. And global variables can have arbitrarily
complex types, so in theory we can fully exercise the type system,
record layout, and data formatters for native PDB files and PE/COFF
executables on any host platform, as long as our type does not require a
dynamic initializer.

This patch adds basic support for finding variables by name, and adds an
exhaustive test for fundamental data types and pointers / references to
fundamental data types.

Subsequent patches will extend this to typedefs, classes, pointers to
functions, and other cases.

Differential Revision: https://reviews.llvm.org/D53731