gdb: Fix issue with Clang CLI macros

gdb: Fix issue with Clang CLI macros

Clang up to version 15 (current) adds macros that were defined in the
command line or by "other means", according to the Dwarf specification,
after the last DW_MACRO_end_file, instead of before the first
DW_MACRO_start_file, as the specification dictates.  When GDB reads the
macros after the last file is closed, the macros never end up "in scope"
and so we can't print them.  This has been submitted as a bug to Clang
developers (https://github.com/llvm/llvm-project/issues/54506), and PR
macros/29034 was opened for GDB to keep track of this.

Seeing as there is no expected date for it to be fixed, add a workaround
for all current versions of Clang.  The workaround detects when
the main file would be closed and if the producer is Clang, and turns
that operation into a noop, so we keep a reference to the current_file
as those macros are read.

A test case was added to confirm the functionality, and the KFAIL for
running gdb.base/macro-source-path when using clang.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29034
Approved-By: Simon Marchi <simon.marchi@efficios.com>