[COFF] Implement llvm.global_ctors priorities for MSVC COFF targets
Summary:
MSVC and LLD sort sections ASCII-betically, so we need to use section
names that sort between .CRT$XCA (the start) and .CRT$XCU (the default
priority).
In the general case, use .CRT$XCT12345 as the section name, and let the
linker sort the zero-padded digits.
Users with low priorities typically want to initialize as early as
possible, so use .CRT$XCA00199 for prioties less than 200. This number
is arbitrary.
Implements PR38552.
Reviewers: majnemer, mstorsjo
Subscribers: hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D51820
llvm-svn: 341727