appveyor: Upgrade to gcc for mingw 6.3.0

This commit sort of brings back #40777 by upgrading back to 6.3.0. While
investigating #40546 it was discovered that 6.3.0 appears to not spurious
fail in the same way that 6.2.0 does (which we're currently using). The
workaround for #40184 contained in #40777 did not work so this commit also
contains a different workaround for the gdb issue. We will not download the
6.2.0 version of gdb and use that instead of the default version that comes with
6.3.0.

I'm going to optimistically say...

Closes #40546

appveyor.yml

@@ -35,13 +35,13 @@ environment:
     RUST_CONFIGURE_ARGS: --build=i686-pc-windows-gnu --enable-ninja
     SCRIPT: python x.py test
     MINGW_URL: https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror
-    MINGW_ARCHIVE: i686-6.2.0-release-posix-dwarf-rt_v5-rev1.7z
+    MINGW_ARCHIVE: i686-6.3.0-release-posix-dwarf-rt_v5-rev2.7z
     MINGW_DIR: mingw32
   - MSYS_BITS: 64
     SCRIPT: python x.py test
     RUST_CONFIGURE_ARGS: --build=x86_64-pc-windows-gnu --enable-ninja
     MINGW_URL: https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror
-    MINGW_ARCHIVE: x86_64-6.2.0-release-posix-seh-rt_v5-rev1.7z
+    MINGW_ARCHIVE: x86_64-6.3.0-release-posix-seh-rt_v5-rev2.7z
     MINGW_DIR: mingw64
 
   # 32/64 bit MSVC and GNU deployment
@@ -60,14 +60,14 @@ environment:
     RUST_CONFIGURE_ARGS: --build=i686-pc-windows-gnu --enable-extended --enable-ninja
     SCRIPT: python x.py dist
     MINGW_URL: https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror
-    MINGW_ARCHIVE: i686-6.2.0-release-posix-dwarf-rt_v5-rev1.7z
+    MINGW_ARCHIVE: i686-6.3.0-release-posix-dwarf-rt_v5-rev2.7z
     MINGW_DIR: mingw32
     DEPLOY: 1
   - MSYS_BITS: 64
     SCRIPT: python x.py dist
     RUST_CONFIGURE_ARGS: --build=x86_64-pc-windows-gnu --enable-extended --enable-ninja
     MINGW_URL: https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror
-    MINGW_ARCHIVE: x86_64-6.2.0-release-posix-seh-rt_v5-rev1.7z
+    MINGW_ARCHIVE: x86_64-6.3.0-release-posix-seh-rt_v5-rev2.7z
     MINGW_DIR: mingw64
     DEPLOY: 1
 
@@ -96,6 +96,26 @@ install:
   - if defined MINGW_URL 7z x -y %MINGW_ARCHIVE% > nul
   - if defined MINGW_URL set PATH=%CD%\%MINGW_DIR%\bin;C:\msys64\usr\bin;%PATH%
 
+  # Here we do a pretty heinous thing which is to mangle the MinGW installation
+  # we just had above. Currently, as of this writing, we're using MinGW-w64
+  # builds of gcc, and that's currently at 6.3.0. We use 6.3.0 as it appears to
+  # be the first version which contains a fix for #40546, builds randomly
+  # failing during LLVM due to ar.exe/ranlib.exe failures.
+  #
+  # Unfortunately, though, 6.3.0 *also* is the first version of MinGW-w64 builds
+  # to contain a regression in gdb (#40184). As a result if we were to use the
+  # gdb provided (7.11.1) then we would fail all debuginfo tests.
+  #
+  # In order to fix spurious failures (pretty high priority) we use 6.3.0. To
+  # avoid disabling gdb tests we download an *old* version of gdb, specifically
+  # that found inside the 6.2.0 distribution. We then overwrite the 6.3.0 gdb
+  # with the 6.2.0 gdb to get tests passing.
+  #
+  # Note that we don't literally overwrite the gdb.exe binary because it appears
+  # to just use gdborig.exe, so that's the binary we deal with instead.
+  - if defined MINGW_URL appveyor-retry appveyor DownloadFile %MINGW_URL%/2017-04-20-%MSYS_BITS%bit-gdborig.exe
+  - if defined MINGW_URL mv 2017-04-20-%MSYS_BITS%bit-gdborig.exe %MINGW_DIR%\bin\gdborig.exe
+
   # Otherwise pull in the MinGW installed on appveyor
   - if NOT defined MINGW_URL set PATH=C:\msys64\mingw%MSYS_BITS%\bin;C:\msys64\usr\bin;%PATH%