FASTBuild: Fix default MSVC compiler PDB paths

When we pass a PDB output directory to the compiler in a path that
requires quoting, the trailing backslash must be escaped to be parsed
correctly by the compiler, e.g., `cl /Fd"path\with space\\"`.  However,
`fbuild` does not parse this correctly when extracting `/Fd`.  Work
around that bug by using a trailing forward slash in quotes instead.