Identified with misc-include-cleaner.

LLVM is inconsistent about how it converts `errno` to `std::error_code`.
This can cause problems because values outside of `std::errc` compare
differently if one is system and one is generic on POSIX systems.

This is even more of a problem on Windows where use of the system
category is just wrong, as that is for Windows errors, which have a
completely different mapping than POSIX/generic errors. This patch fixes
one instance of this mistake in `JSONTransport.cpp`.

This patch adds `errnoAsErrorCode()` which makes it so people do not
need to think about this issue in the future. It also cleans up a lot of
usage of `errno` in LLVM and Clang.

In a04879ce7dd6, dsymutil switched from using TempFile to
createTemporaryFile. This caused a regression because the two use
different permissions:

 - TempFile opens the file as all_read | all_write
 - createTemporaryFile opens the file as owner_read | owner_write

The latter turns out to be problematic for dsymutil because it either
promotes the temporary to a proper output file, or it would pass it to
`lipo` to create a universal binary and `lipo` preserves the permissions
of the input files. Either way, this caused issues when the build system
was run as a different user than the one ingesting the resulting
binaries.

I did some version control archeology and I couldn't find evidence that
these permissions were chosen purposely. Both could be considered
reasonable default.

This patch changes the permissions to `all read | all write` to make the
two consistent and match the one currently used by the higher level
abstraction (TempFile).

rdar://123722848

This fixes a crash where `path::parent_path` causes an invalid access on
a string upon receiving a path that consists of a single colon.

On Windows machine, with runtime checks enabled build, upon `clang -I:
test.cc` produces:
```
Assertion failed: Index < Length && "Invalid index!", file llvm\include\llvm/ADT/StringRef.h, line 232
...
 #6 0x00007ff7816201eb `anonymous namespace'::parent_path_end llvm\lib\Support\Path.cpp:144:0
 #7 0x00007ff781620135 llvm::sys::path::parent_path(class llvm::StringRef, enum llvm::sys::path::Style) llvm\lib\Support\Path.cpp:470:0
```

Ideally, we can look for the last colon starting from the last
character, but we can instead start from second to last, and handle
empty paths by abusing `0 - 1 == npos`.

startswith/endswith wrap starts_with/ends_with and will eventually go away (to more closely match string_view)

In preparation for removing the `#include "llvm/ADT/StringExtras.h"`
from the header to source file of `llvm/Support/Error.h`, first add in
all the missing includes that were previously included transitively
through this header.

Differential Revision: https://reviews.llvm.org/D149513

Differential Revision: https://reviews.llvm.org/D137800