Changes

[SLP] add test for miscompile with reduction (PR43948); NFC

clang/Modules: Error if ReadASTBlock does not find the main module
If ReadASTBlock does not find its top-level submodule, there's something 
wrong the with the PCM.  Error in that case, to avoid hitting problems 
further from the source.
Note that the Swift compiler sometimes hits a case in 
CompilerInstance::loadModule where the top-level submodule mysteriously 
does not have Module::IsFromModuleFile set.  That will emit a confusing 
warn_missing_submodule, which was never intended for the main module. 
The recent audit of error-handling in ReadAST may have rooted out the 
real problem.  If not, this commit will help to clarify the real 
problem, and replace a confusing warning with an error pointing at the 
malformed PCM file.
We're specifically sniffing out whether the top-level submodule was 
found/processed, in case there is a malformed module file that is 
missing it.  If there is an error encountered during ReadSubmoduleBlock 
the return status should already propagate through.  It would be nice to 
detect other missing submodules around here to catch other instances of 
warn_missing_submodule closer to the source, but that's left as a future 
exercise.
https://reviews.llvm.org/D70063

[lit] Better/earlier errors for empty runs
Fail early, when we discover no tests at all, or filter out all of them.
There is also `--allow-empty-runs` to disable test to allow workflows 
like `LIT_FILTER=abc ninja check-all`.  Apparently `check-all` invokes 
lit multiple times if certain projects are enabled, which would produce 
unwanted "empty runs". Specify via `LIT_OPTS=--allow-empty-runs`.
There are 3 causes for empty runs: 1) No tests discovered.  This is
always an error.  Fix test suite config
  or command line. 2) All tests filtered out.  This is an error by
default, but can be
  suppressed via `--alow-empty-runs`.  Should prevent accidentally
  passing empty runs, but allow the workflow above. 3) The number of
shards is greater than the number of tests.  Currently,
  this is never an error.  Personally, I think we should consider
  making this an error by default; if this happens, you are doing
  something wrong. I added a warning but did not change the behavior,
  since this warrants more discussion.
Reviewed By: atrick, jdenny
Differential Revision: https://reviews.llvm.org/D70105

Performance: Add a set of visited SymbolFiles to the other FindFiles
variant.
This is basically the same bug as in r260434.
SymbolFileDWARF::FindTypes has exponential worst-case when digging 
through dependency DAG of .pcm files because each object file and .pcm 
file may depend on an already-visited .pcm file, which may again have 
dependencies. Fixed here by carrying a set of already visited 
SymbolFiles around.
rdar://problem/56993424
Differential Revision: https://reviews.llvm.org/D70106

[Hexagon] Update PS_aligna with max stack alignment once isel completes

[libcxxabi] Prevent cmake from removing our explicit system C++ include
paths
We build with `-nostdinc++` and add our own header path via
`LIBCXXABI_LIBCXX_INCLUDES`.  However cmake tried to be clever and if
`LIBCXXABI_LIBCXX_INCLUDES` happens to match the compilers system path 
it will remove the `-I` flag meaning we can't access any C++ headers.
Ideally cmake would be able see that we are using `-nostdinc++` and 
disable this behaviour.
Differential Revision: https://reviews.llvm.org/D69973

[OPENMP]Use copy constructors instead of assignment operators in declare
reduction initializers.
Better to use copy constructor at the initialization of the declare 
reduction construct rather than assignment operator.