Fix decl order for DeclsInPrototype
Fix decl order for DeclsInPrototype

When a declarator is constructed for a function prototype in
`Parser::ParseFunctionDeclarator`, it calls getCurScope()->decls()
in order to populate DeclsInPrototype.

getCurScope()->decls() return a range from a llvm::SmallPtrSet, which
doesn't guarantee any order. Later on, DeclsInPrototype are used to
populate decl contexts that end up being serialized when PCH or modules
are used. This causes non-determinism in module serialization, which
is bad for lots of reason, including that when a PCH used Modules, it
can't rebuild the module if there's a signature mismatch.

This patch fixes that by sorting out the result getCurScope()->decls()
before populating DeclsInPrototype. I don't believe we can change this
data structure because it can be used for any type of scope (not only
function prototypes).

There are no testcases for this change, since you have to run the
problematic non-reducible testcase multiple times to get a mismatch.
However, one can try to reproduce it on macOS using:

echo '@import Foundation;' > test.m
clang -fmodules test.m -o test.o -c \
      -fmodules-cache-path=/tmp/cache \
      -isysroot
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk
mv /tmp/cache /tmp/cache1

clang -fmodules test.m -o test.o -c \
      -fmodules-cache-path=/tmp/cache \
      -isysroot
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk
mv /tmp/cache /tmp/cache2

HASH=`ls /tmp/cache1`
for i in `find /tmp/cache1 -type f -iname "*.pcm"`; do
  F=`basename $i`;
  diff /tmp/cache1/$HASH/$F /tmp/cache2/$HASH/$F;
done

rdar://problem/43442957
rdar://problem/46722110

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang