folly: windows: inline debug info into .obj rather than use .pdb

Summary:
We see flakey builds to conflicting writes to .pdb files; the error
message suggests enabling `/FS` which is already enabled.  Some serious
internet research reveals that we can put the debug info into `.obj` files
instead of having everyone contend with the same `.pdb` file.

```
C:\PROGRA~2\MIB055~1\2017\BUILDT~1\VC\Tools\MSVC\1416~1.270\bin\Hostx64\x64\cl.exe  /nologo /TP -DBOOST_CONFIG_SUPPRESS_OUTDATED_MESSAGE -DGFLAGS_IS_A_DLL=1 -DWIN32_LEAN_AND_MEAN -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_WARNINGS -D_ENABLE_EXTENDED_ALIGNED_STORAGE -D_SCL_SECURE_NO_WARNINGS -D_STL_EXTRA_DISABLED_WARNINGS="4774 4987" -IZ:\shipit\folly -I. -IZ:\installed\boost-6XBjAEHS6zoZVAY7uQHhSxYamF-vUNWngO1YaEIiiko\include\boost-1_69 -IZ:\installed\double-conversion-8uX8TcZDEz7gXyY70bMXqPUF2uLqH2OTWBffkLfU7FU\include -IZ:\installed\gflags-Md0yNU5drHUQ6MCdsB6iC0WS5YX-RFZk_eTHNKjkcPs\include -IZ:\installed\glog-_3NtzkcZDkkSFkgC70vwofqrnao05-mPxEa0E546a9g\include -IZ:\installed\libevent-TIUwVyvnVZZVHDsesjdIidM_iH4er-_6pEE0X4dzuL4\include -IZ:\installed\openssl-0G_glZ7y5BplM752sREIHKuam0GcoPqw7m6yjEGefyE\include -IZ:\installed\zlib-lxujVMVR4hhdh4-S0SCxkb55DAWh5ISCLWbfL7m7hwg\include -IZ:\installed\zstd-M8csiME1A2X7ZGECWeSGmWbqM1glHd6xoskhyPHgpaw\include -IZ:\installed\snappy-Z3WzzGXxgX1-6zSn4NdRg8Z3coQaz7Gob3gGoomN95E\include -IZ:\installed\fmt-W9-akKzyOlf4GoHcFsw1_rCxt3y0ghyjO3gb5Xve1Gc\include /DWIN32 /D_WINDOWS /W3 /GR  /MD /Zi /O2 /Ob1 /DNDEBUG   /EHs /GF /Zc:referenceBinding /Zc:rvalueCast /Zc:implicitNoexcept /Zc:strictStrings /Zc:threadSafeInit /Zc:throwingNew /permissive- /std:c++latest /bigobj /favor:blend /Zc:inline /Wall /MP /Gw /Gy /Qpar /Oi /Ot /wd4191 /wd4291 /wd4309 /wd4310 /wd4366 /wd4587 /wd4592 /wd4628 /wd4723 /wd4724 /wd4868 /wd4996 /wd4068 /wd4091 /wd4146 /wd4800 /wd4018 /wd4365 /wd4388 /wd4389 /wd4100 /wd4459 /wd4505 /wd4701 /wd4702 /wd4061 /wd4127 /wd4200 /wd4201 /wd4296 /wd4316 /wd4324 /wd4355 /wd4371 /wd4435 /wd4514 /wd4548 /wd4571 /wd4574 /wd4582 /wd4583 /wd4619 /wd4623 /wd4625 /wd4626 /wd4643 /wd4647 /wd4668 /wd4706 /wd4710 /wd4711 /wd4714 /wd4820 /wd5026 /wd5027 /wd5031 /wd5045 /we4099 /we4129 /we4566 /showIncludes /FoCMakeFiles\folly_base.dir\folly\Format.cpp.obj /FdCMakeFiles\folly_base.dir\ /FS -c Z:\shipit\folly\folly\Format.cpp

Z:\shipit\folly\folly\Format.cpp: fatal error C1041: cannot open program database 'Z:\build\folly\CMakeFiles\folly_base.dir\vc140.pdb'; if multiple CL.EXE write to the same .PDB file, please use /FS

ninja: build stopped: subcommand failed.
```

Reviewed By: yfeldblum

Differential Revision: D20752325

fbshipit-source-id: 4f20871d54cc0973b3d025734dd0886c202c839b

CMake/FollyCompilerMSVC.cmake

@@ -88,6 +88,22 @@ foreach(flag_var CMAKE_C_FLAGS_DEBUG CMAKE_CXX_FLAGS_DEBUG)
   endif()
 endforeach()
 
+# When building with Ninja, or with /MP enabled, there is the potential
+# for multiple processes to need to lock the same pdb file.
+# The /FS option (which is implicitly enabled by /MP) is widely believed
+# to be the solution for this, but even with /FS enabled the problem can
+# still randomly occur.
+# https://stackoverflow.com/a/58020501/149111 suggests that /Z7 should be
+# used; rather than placing the debug info into a .pdb file it embeds it
+# into the object files in a similar way to gcc/clang which should reduce
+# contention and potentially make the build faster... but at the cost of
+# larger object files
+foreach(flag_var CMAKE_C_FLAGS_DEBUG CMAKE_CXX_FLAGS_DEBUG)
+  if (${flag_var} MATCHES "/Zi")
+    string(REGEX REPLACE "/Zi" "/Z7" ${flag_var} "${${flag_var}}")
+  endif()
+endforeach()
+
 # Apply the option set for Folly to the specified target.
 function(apply_folly_compile_options_to_target THETARGET)
   # The general options passed: