cmake: fix MSVC PCH support

I'm not sure if the previous implementation ever worked.

Source/PCH/CMakeLists.txt

@@ -1,6 +1,24 @@
+# The PCH that dolphin uses for MSVC is non-standard;
+# Instead of having one PCH per module, dolphin has one PCH shared between all modules.
+# So we need to implement PCH manually, rather than using the PCH support built into cmake
+
 add_library(pch pch.h pch.cpp)
-set(PCH_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
-set(PCH_NAME ${PCH.pch})
-target_compile_options(pch PUBLIC /Ycpch.h /Fp${PCH_DIRECTORY}/${PCH_NAME})
+
+# pch.cpp should be compiled with the /Yc command, which creates the precompiled header
+target_compile_options(pch PRIVATE /Yc"pch.h" /Fo$<TARGET_FILE_DIR:pch>)
+
+# /Fp sets the location of the PCH. By forcing it to a fixed location, all modules
+# will share this one PCH
+target_compile_options(pch PUBLIC /Fp$<TARGET_FILE_DIR:pch>)
+
+# Sharing a PCH breaks pdb files. So we use the /Z7 option to inline the pdb into
+# the binary. That also requires us to disable minimal rebuilds.
+target_compile_options(pch PUBLIC /Z7 /Gm-)
+
+# targets which include pch need these compile options
+# /Yu - Use precompiled header named "pch.h"
+# /FI - Force include "pch.h" at top of every source file
+target_compile_options(pch INTERFACE /Yu"pch.h" /FI"pch.h")
+
 # fmt/format.h is included in the PCH
 target_link_libraries(pch PUBLIC fmt::fmt)

Source/PCH/nopch/pch.h

@@ -0,0 +1,4 @@
+// dummy include to help with disabling pch for a single file
+// cmake doesn't provide a clean way to disable MSVC's force include option
+
+// So we can just point it at an empty file instead.