Changes

[PowerPC][XCOFF] Add support for zero initialized global values.
For XCOFF, globals mapped into the .bss section are linked as COMMON 
definitions. This behaviour is incorrect for zero initialized data, so 
emit those to the .data section instead.
Differential Revision: https://reviews.llvm.org/D69528

clang/Modules: Use range-based for in ASTReader::ReadAST, NFC

clang/Modules: Split loop in ReadAST between failable and not
Split a loop in ReadAST that visits the just-loaded module chain, 
between an initial loop that reads further from the ASTs (and can fail) 
and a second loop that does some preloading (and cannot fail).  This 
makes it less likely for a reading failure to affect the AST.
This is not fixing a known bug and the behaviour change may not be 
observable, it's just part of an audit to look at all of the error 
handling in the ASTReader.
https://reviews.llvm.org/D70056

[X86] Autogenerate checks in fp-intrinsics.ll. Split fma tests off to
fp-intrinsics-fma.ll. NFC

[X86] Add avx512 command lines to vector-constrained-fp-intrinsics.ll.
NFC

[lldb] Re-enable VSCode tests
The VSCode tests were all disabled on macOS because the implementation 
had some issues that resulted in flakiness on Darwin. It seems most of 
these issues have been addressed. I've re-enabled all the tests that 
consistently passed locally.

clang/Modules: Add missing diagnostics for malformed AST files
These were found via an audit.  In the case of `ParseLineTable` this is 
actually dead code, since parsing the line table always succeeds, but 
it's prudent to be defensive since it's possible an assertion there 
could be converted to a `true` return in the future.